BOSTON — Early Saturday as the Triple-A Worcester Red Sox prepared to play a doubleheader at home, manager Chad Tracy called Marcelo Mayer into his office. He didn’t have Mayer in the lineup for the first game. The reason? The big-league club was awaiting word on third baseman Alex Bregman’s MRI. An injury might create a spot on the MLB roster.

Not long after, Mayer got a second call into Tracy’s office.

“He said something along the lines of, ‘You’re not playing. You’ll play the second game, but not here. You’ll be playing in Fenway,’” Mayer, 22, said. “I just got the craziest rush through my whole body. It was a pretty cool thing.”

Mayer, the No. 28 prospect in baseball according to The Athletic’s Keith Law, was about to make his major-league debut. But first, he’d have to find his keys.

With the Red Sox playing in a doubleheader of their own against the Baltimore Orioles, Mayer rushed to pack his bags to make the 45-mile drive from Polar Park in Worcester to Fenway Park but realized he had no way to get there.

“I lost them, like, three weeks ago, and I never cared to look for them until I needed to,” Mayer lamented, as he stood in the Red Sox clubhouse moments after he’d arrived in Boston roughly 90 minutes before the first pitch of the second game.

Mayer left his car at Polar Park and one of the Worcester clubhouse assistants volunteered to drive him to Boston. Mayer had called his parents as soon as he heard about the promotion but spent the entire car ride buried in his phone, replying to a flood of texts congratulating him on his impending debut.

By the time they got off the highway and into the city, though, the crowds around Fenway were thick as fans waited to enter for the second game.

“Honestly, I was just looking at my phone, so I kind of barely realized it,” he said. “But the clubby, he was making jokes to get a police escort, to get everybody out of the way, because he was getting more nervous.”

Needless to say, it was a chaotic day leading up to his debut and that was before the standing ovation when he stepped to the plate in the second inning.

A large portion of the 34,604 fans in the Fenway crowd rose from their seats to watch Mayer take his first at-bat. He struck out looking on six pitches.

“I’ve never experienced anything like that in my life,” he said after Boston’s 2-1 loss to Baltimore. “Actually, I just got done texting my friends and parents about that moment. It’s something I’ll remember the rest of my life.”

Mayer started at third base, a position he’d played at just six times professionally before Saturday, and batted sixth. He went 0-for-4 with three strikeouts and reached on a fielder’s choice in the ninth.

After the Red Sox pinch hit for shortstop Trevor Story in the seventh inning, Mayer moved to short in the eighth inning but then shifted back to third in the ninth when David Hamilton entered as a pinch runner and took over at short. Mayer is expected to see time at second, third and short as the Red Sox mix and match in the infield without Bregman.

“My job here is to do whatever I can help the team win,” Mayer said. “I’m gonna go out every single day, give it 100 percent, give it my all, and hopefully that’s enough.”

His day at the plate wasn’t what he’d wanted, but despite all the chaos leading up to it, he said he was calm.

“I wouldn’t say I was nervous,” he said. “I just feel like I tried to do too much. I feel like next time I’ve got to do a better job with that. But with that being said, I’ve never experienced anything like that (game). So it was really fun.”

Having spent all of spring training with the big-league club, Mayer and fellow top prospects Roman Anthony and Kristian Campbell grew close with their big-league teammates. When Mayer arrived Saturday, there was a bittersweet juxtaposition in the clubhouse as players hugged him and clapped his back, excited for his promotion while also acknowledging the loss of Bregman, who spoke quietly with the media about a severe quad injury. Bregman missed 58 games in 2021 with a similar injury.

As the Red Sox plan to be without Bregman for an extended period, Mayer figures to be a key part of the club.

Mayer undoubtedly had pictured his big-league debut countless times over the years. How it played out in reality Saturday wasn’t quite what he’d envisioned. He didn’t get the big hit, and the Red Sox as a whole failed to capitalize on seven scoreless innings from starter Lucas Giolito in the loss.

But after years of waiting, Mayer has a new home in Boston and plenty of work ahead of him.

“It’s the big leagues,” manager Alex Cora said of Mayer’s day. “Great experience for him (today). Played good defense, obviously, offensively not much happened, but he’s a big leaguer now, can’t take that away from him.”