BOSTON -- The New York Yankees opened their first series of the season at Fenway Park without two of their biggest bats in the starting lineup: All-Star outfielder Aaron Judge and slugging first baseman Anthony Rizzo.

Judge did not start Thursday night in the team's 6-5 win over the Boston Red Sox due to what manager Aaron Boone described as "lower body stiffness."

"The last couple of nights [he's] just been dealing with some tightness in his lower half, so just felt like today was the day that I needed to have him down. The idea is for him to be back in there tomorrow, but [it's] not something I'm overly concerned about," Boone said. "But, more importantly, just want to be something that we get out ahead of and make sure that it doesn't turn into something big. He's been able to kind of navigate through these last couple of days but, especially getting in as late as we did last night, just not something I want to mess with right now."

Asked to further elaborate on what Judge was feeling, Boone described it as "just calves."

"Just gets a little heavy and tight more than anything. Just something that I don't want to mess with, especially where we are in the season, I don't want to become a larger issue so hopefully something we'll get out ahead of a little bit," he said.

Judge's comments after the game echoed what Boone said earlier in the day.

"It's just precautionary stuff; been feeling it for a week or so," he said. "Just taking the chance to take a day and make sure everything's all right; make sure it's nothing more than one day so we can keep rolling."

Judge expressed confidence he would be back in the lineup Friday in Boston.

"We talked about it a couple of days ago and just kind of tried to map it out," he said. "Kind of figured out what would be the best time and place, and today worked out. And now it's time to get back to work."

The Yankees, who are toward the end of a four-city, 11-day, 10-game road trip, had a late-night arrival in Boston after a lopsided 16-0 win over the Pirates on Wednesday night, a game that was delayed for over an hour by inclement weather. The Yankees split the two-game series in Pittsburgh, with Rizzo out of the lineup for both matchups due to "lower back stiffness."

The first baseman said he was feeling much better despite not being in the lineup for a third straight game, describing it as a type of lower back spasm that has been a recurring issue in his career.

"I've dealt with this in years past, and I just feel like we've been ahead of it big time. Yesterday, I made a lot of progress and today I'm making a lot of progress," Rizzo said of the spasm, which he said tends to resolve itself in 6-8 days. "I went on the IL for it once... that was in April [of 2018] to be super cautious just because it was April. I don't foresee that happening now. I think it's just day-to-day. Even today, it's way better than it was yesterday. Back pain is the worst but the progress we've made with the treatment is great."

The Yankees came into the four-game set at Fenway holding a 14-game lead atop the AL East, giving them enough leeway to not miss two of their key sluggers. Boone said that if the game was at a crucial point in the season, both Judge and Rizzo would undoubtedly be in the starting lineup.

Judge hit his Major League-leading 30th home run of the season and his third career grand slam Wednesday at Pittsburgh. He also leads all of MLB in runs scored (65) and ranks second in slugging percentage (.627). Rizzo's 22 homers this season are tied for fifth in the majors, and alongside Judge and Giancarlo Stanton (21 HRs) they became the first trio in franchise history to each hit at least 20 home runs in the club's first 79 games of a season.

While Boone said Judge could have been an option off the bench, the Yankees planned to stay away from Rizzo, though the first baseman said he hoped to able to play at some point during the series.

"The good thing is we have put ourselves in a really nice position that we don't have to worry about the emergencies and things like that," Rizzo said. "Not saying that we're not going to keep putting the full foot to the throttle. But when you do have cases like this, it's definitely a little easier than that stressful, every-game mentality."