New Jets coach Aaron Glenn needs no introduction -- not to Jets fans, at least.

From 1994 to 2001, Glenn was a Jet. Selected with the No. 12 pick in the 1994 draft, the scrappy corner from Texas A&M had the type of unstable experience with the franchise that he intends to end as its new head coach, with a carousel of changing schemes, leadership styles and philosophies. He played for five different head coaches over his eight years in New York, and the organization has had another five since.

Glenn knows what it means to be a Jet -- to deal with the media circus and filter out criticism. He's as equipped to deal with that scrutiny as any 15-year NFL vet should be -- that career length ties Jim Harbaugh for the longest playing career of any current head coach. But there are other advantages that come with playing for so long in the NFL and for spending so long with a franchise as tumultuous as the Jets. He played for a lot of coaches in a lot of systems.

During his time with the Jets, Glenn was coached by Pete Carroll, Bill Parcells, Bill Belichick, Mike Nolan, Todd Bowles and Herm Edwards. He was then selected by the Texans, led by Dom Capers and Vic Fangio, as part of the 2002 expansion draft. With the Cowboys in 2005 and 2006, he was back under Parcells, but with Mike Zimmer as his defensive coordinator. That's also where he met Sean Payton, who would become the head coach of the Saints, for whom Glenn played in 2008. Glenn played there under secondary coach Dennis Allen, who would later employ Glenn as his defensive backs coach in New Orleans from 2016 to 2020. 

A defender at the turn of the century couldn't ask for a better curriculum than to play under Parcells, Belichick, Capers, Fangio and Zimmer. Short of a stint in Tampa Bay with Monte Kiffin, Glenn played under the Mount Rushmore of 2000s defenses.

There are a couple of things that come with playing for so many coaches. The first is an appreciation for technique -- the connective tissue between all defensive schemes, 3-4 or 4-3, man or zone, single- or two-high. A backpedal is still a backpedal, the half-turn is still the half-turn.

"He was a technician," said Capers, Glenn's coach in Houston from 2002 to 2004. "He not only knew what to do, but he was very involved in the basics of how to do it."

The second is perhaps more pertinent for Glenn's success as a coordinator and future as a coach: With tons of reps in tons of systems comes an appreciation for how schemes fit players and how players fit schemes. Many head coaches had little to no playing experience at the NFL level, doing all of their learning in the coaching ranks. Some bounced from team to team, but the NFL is a connections business, so many stayed within the same schematic family as they climbed the ranks -- that's who was offering them jobs, after all -- the coaches they already knew. They spoke one language, sharpened their tools and innovated only when environmental pressures demanded it.

In contrast, Glenn spent 15 years watching different coaches maximize different players. And not just watching but living it, too. He was a 5-foot-9 corner who was so far outside of Parcells' ideal measurements that he stormed into his coach's office in their first season together and demanded to know if he was going to be traded. (He wasn't, and he made the Pro Bowl that season -- and the next.) No, Glenn wasn't the size Parcells wanted for his defense -- Belichick, Parcells' defensive coordinator who had been head coach of the Browns in 1994, had passed on him in the draft. But Parcells and Belichick coached around Glenn. That's what good coaches do.

"Some of the best coaches I've ever been around always looked at it like this: You look at your personnel, and you try to design a defense that fits your personnel," Glenn told ESPN over the Lions' wild-card bye week.

Glenn had four years as the Lions' defensive coordinator, and his personnel changed dramatically. The only players who took significant snaps on both the 2024 and 2021 units were players who were acquired in 2021, during Glenn's first offseason: free agent linebacker Alex Anzalone (who followed him from New Orleans), defensive tackles Alim McNeill and Levi Onwuzurike, linebacker Derrick Barnes and defensive back Ifeatu Melifonwu. And because there has been such substantial personnel turnover, it's easy to track how Glenn has fit his scheme to the personnel -- how he has done that thing the best coaches do.

Glenn's influences, and why he was afforded patience

Start in 2021, when Glenn was hired to be a first-time coordinator under a first-time, kneecap-biting head coach for a joke of a franchise harboring no national hype or lofty expectations. The eyes of the world were not yet on the Lions, but Glenn had been preparing for this opportunity for years.

"The first day I went into Detroit, we looked at all the cut-ups that he had from when he was a secondary coach at New Orleans," said Capers, whom Glenn hired as a senior defensive assistant. "He had everything cut up, from a technique standpoint, from a scheme standpoint -- it was all right there on his computer. Any time we were installing something new, we could go back and refer to those cut-ups and see exactly what AG was talking about and what he wanted to do."

While all of the technique, drills and "teach tape" were edited from Glenn's New Orleans days, the Lions couldn't copy-paste the Saints' defense. "If you go back to our first year here, we had smaller outside 'backers," Glenn said. "We were more of a true 3-4, and we were playing more quarter-quarter-half, because that's who we had in our secondary."

The Saints had run a 4-3, with supersized defensive ends Cameron Jordan and Marcus Davenport securing the edge. The lighter, leaner edge defenders in Detroit necessitated a schematic switch -- the design of the defense needed to change to fit the personnel.

Glenn's 3-4 that first season was more reminiscent of Mike Pettine's defense, from when Glenn coached with the Browns (2014-15) and Rex Ryan's from when Glenn was a personnel scout with the Jets (2012-13). It was also influenced by Capers and Parcells. Remember: a lot of coaches, a lot of systems.

Here's what that 3-4, two-high structure looked like in Week 2 against the Packers in 2021.

That defense had a lot of problems. The Lions were making the best of the personnel left over from the failed Matt Patricia era -- they went 14-33-1 from 2018 to 2020 -- and Glenn was a first-time playcaller trying to figure out the right system for those mismatched pieces. For example, the Packers attacked the Lions' base 3-4 with "slot" formations in which the two wide receivers are on the same side of the field. These formations displaced an outside linebacker off the line of scrimmage while yanking the now unoccupied cornerback into the box as a quasi-linebacker. Watch how easily the Packers found space by manipulating that outside linebacker.

Glenn remembers these plays, when he lost structurally, as lessons: "Sean Payton used to always tell me with his process, the first thing he used to look at was how would a team operate in base personnel against slot. Corners over? Corners stay?"

"So I started to think about how all these offensive coordinators think. What are they looking for? How are they trying to make sure that they can attack me on some of the things that I run?"

This change -- thinking about his defense not from a defensive perspective, but from an offensive perspective -- provided what Glenn called his biggest growth in his four years as a defensive coordinator.

It has not happened by accident or overnight -- nothing Glenn does is without intention, and nothing he says is lip service. He has spent every offseason since 2021 meeting with offensive coordinators to learn how they build game plans, collegiate defensive coordinators to see what schematic wrinkles he can glean, and veteran coaches to prepare for end-of-game situations.

The Jets have long employed, for better or worse, coaches entrenched in their approaches. In Glenn, there is a relentless curiosity, a compulsive and almost subconscious gathering of information. He was last with the Jets in 2012 and 2013 as a member of the scouting department, where he was competing not against former NFL players, but aspiring general managers with an entirely different set of skills. He taught himself Excel, PowerPoint and Visio to catch up to them, to turn reports around just as fast as them.

In New Orleans, he built new rules for stack and bunch coverages that were based on how Jay Wright's Villanova men's basketball teams defended the pick-and-roll -- the Wildcats had good rules on switching defenders and avoiding picks, so he learned and installed them. Payton told Glenn that nugget about hunting a defensive coordinator's tendencies in base personnel against slot, and Glenn squirreled it away. "I want to learn as much as I can," Glenn told The Pivot podcast in 2022, "because I want to beat you."

The slot formation problem was the first of many similar fires Glenn would encounter and then put out through the Lions' arduous, patient rebuild. In 2022, the defensive scheme changed again, as the Lions added some of those big ends Glenn was familiar with from New Orleans -- Aidan Hutchinson and Josh Paschal. They ran far more man coverage then as well, with a healthy Jeff Okudah at corner and a good nickel in Mike Hughes. In 2023, a great safety room and shaky corner play sent them right back to zone defense.

Look at how the rate of man coverage changed under Glenn through four years, culminating with the 2024 season, in which the Lions finally had the corners necessary to play some sticky man-to-man.

Other defensive coaches don't fluctuate like this. They run their stuff. In the modern NFL with its microwave culture, three years of shaky defense feels like an unbearable eternity. Coaches can't spend the time experimenting, discovering and troubleshooting. The Lions were always in it for the long haul, a gradual and sustainable ascent. Glenn took the time afforded him not to microwave, but to cook. To make something a little more substantial.

How Glenn led Detroit's defensive revival

With more in his toolbox this season -- trustworthy corners in Carlton Davis, Terrion Arnold and Amik Robertson; an elite safety duo; and a deep and versatile defensive line -- Glenn was finally equipped to put those schematic lessons to work.

"As I started to [think like an offensive coordinator], I started not giving him exactly what he's looking for, then sometimes giving him what he's looking for, but we're actually doing something totally different," he said.

Here's how that played out in real time. In Week 2, the Tampa Bay offense walked out in a slot formation on fourth-and-2. The Lions were in a 3-4 again, but this time they had the corner over: Instead of asking the outside linebacker to bounce out to the slot, Davis traveled with the wide receiver. It's a clear "man coverage" indicator -- Davis over a receiver and safety Brian Branch over the tight end to the top of the screen.

Buccaneers quarterback Baker Mayfield dropped back, faked the give and flipped his head back to the defense. What does he see? Zone coverage. The Lions dropped Davis into a deep half like he was a safety and dropped the linebackers into the dig windows. Mayfield had nowhere to go with the football: scramble, panic lateral, TFL, turnover on downs. Watch the play:

This is what coaches call "a beater" -- a specific coverage, called in a specific moment, to beat a specific playcall. It's pure outfoxing, and it comes only as the result of Glenn's understanding of the opposing offensive coordinator. On third downs this season, Detroit had the best defense in football, allowing a conversion rate of 32.4%. It ranked second on fourth down (41.4%). That's situational football. That's painting an opponent into a corner, then knowing how it behaves once it's there.

"I want to learn as much as I can, because I want to beat you." Glenn is obsessed with understanding his opponent. It's an obsession so total it informs not just how he prepares, but also his entire defense. His meeting rooms are dangerous places to be.

"He'll give you a situation -- whether it's first/second-down day, or third-down day, whatever," said Trevor Nowaske, a young linebacker thrust into serious playing time this season by injuries above him. "And he'll be like, 'What are we calling here?'"

This cold-calling technique, most commonly used in law schools, has become commonplace in NFL coaching circles. Anyone in the room -- from the 10-year vet and the multimillionaire star to the undrafted practice squad player -- can get put on the spot. Not just for their particular assignment on a play, or an opponent's tendency. Glenn wants his players to have such a firm understanding of the defense's goals -- how it's designed, why it's designed that way, what it exploits in the opponent, when it will be deployed to exploit that one weakness -- that they can predict his exact playcalls. And they can.

"It'll be third-and-something, and your mind will start going, 'Oh, this is the play AG's going to call," Onwuzurike said. "Especially in, like, last five [play] situations, end-of-game situations? I can go down the board."

Added Melifonwu: "Last year was really when I knew why he was calling certain stuff. Sometimes you'll see the down and distance, you'll see the situation in the game, and you already know what he's going to call. It helps. Instead of just lining up and playing, it's like 'OK, he's calling this so I can get ready to do this, or I can get ready to play this leverage.'"

Melifonwu recalls a play on which the lightbulb came on. It was the first drive of the game against the Broncos in the 2023 season. Denver was in the high red zone on second-and-10 in 12 personnel. Russell Wilson went under center, sent a tight end in motion and snapped the football.

Go ahead. Stand up in the meeting room in front of the entire defense and make your playcall prediction. What defense did Glenn dial up here?

The Broncos were play-faking a split zone run to the left to run flood to the right: a three-level, vertical stretch. It's a staple of any Wilson offense. But it has a weakness. Because Wilson had to fake the give to his left before turning around and reading the defense out to his right, he couldn't see any pressure coming off the frontside.

So that's exactly what Glenn called; what Melifonwu expected to hear when he saw the personnel, the field position, the down and distance. Once Melifonwu saw the formation, he knew he was blitzing -- and because Glenn hammered home the why behind the blitz call, Melifonwu knew Wilson wouldn't see him coming:

Of all the prophets Glenn created, Melifonwu is perhaps the most fervent believer, because of his literal conversion. Drafted as a corner in 2021, Melifonwu found himself sitting in Glenn's office during OTAs in 2022. Glenn was thinking about moving him to safety. It was up to Melifonwu, who hadn't played safety since his freshman year of high school and wasn't enthusiastic about a change. But Glenn convinced him, listing the reasons Melifonwu could be a great safety: "My size, coverage ability, and tackling," Melifonwu pauses for a second and smiles at the memory. "And that I was smart. Really smart. Smart enough to play safety."

Of all the talents Glenn has developed during his time with the Lions, this skill is unquestionably the greatest. He has gotten players to excel at positions outside of their expected roles.

The Lions were preposterously, unimaginably injured in 2024. Sports Info Solutions, which tracks games lost to injury weighted by how important the lost players were to their team, graded the Lions as the team most negatively affected by injuries. Almost all of the missed snaps came on the defense.

The following defenders missed multiple games for Detroit: Hutchinson, Paschal, Davis, Melifonwu, Alim McNeill, Marcus Davenport, DJ Reader, Kyle Peko, John Cominsky, Brodric Martin, Mekhi Wingo Derrick Barnes, Alex Anzalone, Malcolm Rodriguez, Jalen Reeves-Maybin, Emmanuel Moseley, Khalil Dorsey and Ennis Rakestraw. 

Every week was a mad dash to plug another hole in the leaky boat. Onwuzurike, the defensive tackle, spent the majority of his time as a big defensive end after Hutchinson went down. Anzalone played all three linebacker positions by the end of the season. Branch, who was the starting nickel last season and had moved to strong safety, went back to the nickel so Robertson, the new starting nickel, could become an outside corner. (That move in particular worked, as Robertson locked down Justin Jefferson for four quarters in Detroit's Week 18 win over Minnesota before he broke his arm in the first quarter against Washington.)

Glenn learned more than just good schemes from Parcells and offensive game-planning from Payton. The Lions under Dan Campbell, just as the Saints and Broncos under Payton, run OTAs like Parcells did. There are no ones and twos; every player is expected to rotate in with the "starters" and take reps alongside different teammates.

"We don't have a certain group that's just playing with each other all the time," Glenn said. "I want guys to know: If this guy's out, you have to go in and you have to perform just like the so-called starter there."

It's why the staff knew it could move Onwuzurike to end with no problem. It's why Anzalone was able to shift his specific linebacking assignments when Barnes went down, and again when Rodriguez was injured after him.

"It's a league of injuries, and you just never know when someone's going to go down," Glenn said. "Put a guy in there, and he has to operate the same way because he's in the same meeting as the rest of those guys. We expect those guys to learn and be able to understand the responsibility.

"I'll tell you what, it gives the players a sense of pride to know that at any given time, you can go in there and you're counted on for the defense to not skip a beat."

In the most injured season of Glenn's tenure with the Lions, his defense was the best it has ever been: top 10 in EPA per play, success rate, drive series conversion rate and points per drive allowed. No beats skipped, indeed.

Will Glenn succeed with the Jets?

In New York, Glenn isn't inheriting a ground-up rebuild like the defense he was given in Detroit. The Jets' defense has been really good. From 2021 until head coach Robert Saleh was fired in early October, the Jets' defense ranked first in points per drive, first in EPA per play and third in success rate allowed. The expectation, however unfair it is, will be that Glenn meets the bar Saleh set as the league's best defense.

Even if Glenn clears that bar, won't the Jets just go through the same thing they did with Saleh? An excellent defensive coach who cannot figure out the quarterback position, who lacks the offensive acumen to develop a young passer on his own, who must cycle through subprime offensive coordinator options since the great offensive minds are already head coaches?

This concern misses the forest for the trees. It's not that Glenn is a great defensive coach; it's that he's a great coach. It's not that Sean Payton and Jay Wright had something to teach Glenn about defensive football -- they were running their own shows, and Glenn gleaned some wisdom to impart into his defense. Parcells didn't just teach Glenn how to run the 3-4 -- Parcells taught him how to prepare for midseason injuries during OTAs. When Melifonwu considered making a switch to safety, Glenn assuaged him the same way Parcells once assured a young Jets corner he wouldn't be traded away for having the wrong measurables. "I'm a coach," Glenn said. "I just happen to be on defense."

Wondering who will call plays, and for which quarterback, is warranted. Wondering what Glenn will ask from star cornerback Sauce Gardner, or how he'll elevate Quinnen Williams, is warranted, too. Glenn won't have the same four-year period to turn things around, either -- not in New York, where the coach's seat gets hotter faster.

Another wondering is warranted: Can any coach be successful with the Jets? New York's 14-season playoff drought is twice as long as the next longest. Four different coaches and seven different starting quarterbacks have failed to break the dam. Why should Glenn be any different? Why should Jets fans have hope again?

It's the same thing that could have been asked of Lions fans when Campbell arrived and turned the keys of the defense over to Glenn. It's the same thing that could have been asked of Commanders fans when they hired a defensive head coach whose unit had just gotten decimated in the playoffs -- then Dan Quinn got rookie quarterback Jayden Daniels, went for a few fourth downs and made the NFC Championship Game.

Franchises are always cursed until they aren't. Defensive coaches are always doubted until they prove coaching is more than the side of the ball on which they cut their teeth.

Forget about rebuilding the Lions like Campbell or fixing the Jets like Parcells. Glenn isn't trying to be anyone who came before him: "Obviously, Parcells had a huge influence on me. Payton had a huge influence on me. Mike Tomlin has a huge influence on me. But at the end of the day, I'm Aaron Glenn. Even though I take bits and pieces of everyone of those coaches as a part of me, I know who I am. The players are going to get an authentic Aaron Glenn. I think that speaks volumes more than anything else."

Jets fans don't need an introduction to the authentic Aaron Glenn. The rest of the league will get one soon enough.