SANTA CLARA, Calif. -- In what is the most significant step he's taken in his recovery from Achilles tendinitis, San Francisco 49ers running back Christian McCaffrey returned to practice.

The Niners officially opened McCaffrey's 21-day practice window on Monday afternoon, which means he has three weeks to prove he's ready to be activated from injured reserve to the 53-man roster.

Sure enough, McCaffrey was on the practice field and warming up with his teammates during a roughly two-minute viewing period open to media. For his teammates, McCaffrey's return was a welcome sight, with one noting that McCaffrey's fresh legs had him moving in fast forward compared to everyone else during a team drill.

"It's such an amazing feeling to see that 2-3 out there doing his extensive warmup and a million high knees he does before practice," left tackle Trent Williams said. "It was great seeing him."

While McCaffrey and the 49ers will have time to see how he fares in practice before putting him back in any games, there has been optimism around the team that he will be available on Sunday against the Tampa Bay Buccaneers so long as this week goes well.

For the better part of the past few weeks, the Niners have pointed to this week for McCaffrey's potential return as he continued to work through the tendinitis that kept him out of the team's first eight games.

Before San Francisco took off on its bye last week, coach Kyle Shanahan again suggested McCaffrey would begin practicing on Monday so long as his simulated practice work last week came with no setbacks.

"As long as it all goes good, hopefully we'll get him back in practice next week," Shanahan said on Oct. 28.

McCaffrey originally went on injured reserve on Sept. 14 after dealing with the tendinitis and a calf injury that began hampering him in early August. While McCaffrey recovered from the calf issue, the Achilles tendinitis lingered and eventually led McCaffrey to travel to Germany to meet with a specialist while the 49ers were in Los Angeles to play the Rams on Sept. 22.

While backup Jordan Mason has fared well in McCaffrey's stead -- Mason is third in the league in rushing yards per game (85.6) and sixth in yards per carry among running backs (5.1) -- the Niners have struggled to finish drives in the red zone. They rank 28th in the NFL in red zone touchdown rate, scoring on 48.6% of drives inside the opponent's 20.

McCaffrey scored 21 total touchdowns in 2023 on his way to Offensive Player of the Year honors.

"[You] could probably put him in any offense, it's going to make them better," Williams said. "Obviously our offense is kind of tailor-made to him and some of his strengths, so definitely going to be a tremendous lift for us."

Shanahan indicated last week that McCaffrey's workload when he returns will also be based on how he looks in practice.

"The expectation is to see how he is when he gets back and then evaluate him as he goes," Shanahan said last week. "Of course, the hope is that everything just goes back to normal and it's perfect. That's what we're all striving for, but you've got to play that out smartly and you've got to evaluate that each day. What we see with our own eyes, what he tells us. That'll be something we constantly are working through."

The 49ers also opened the practice window for offensive lineman Jon Feliciano on Monday. Feliciano has missed the first half of the season with a knee injury. Receiver Jauan Jennings (hip) and kicker Jake Moody (sprained right ankle) also were back on the practice field Monday and Williams offered some hope that linebacker Dre Greenlaw could have his practice window opened soon as he continues to recover from the torn Achilles he suffered in February's Super Bowl.

"Definitely super happy for [McCaffrey] that it turned the corner and he was able to go out and do what he loved again," Williams said. "Even had a glimpse of Greenlaw out there sprinting around with a helmet on. Both of them, just seeing that got our juices going."