The Boston Red Sox promoted one of baseball’s best infield prospects to the majors on Saturday. Marcelo Mayer, the fourth pick in the 2021 draft, has joined the big-league club after veteran Alex Bregman landed on the injured list with what manager Alex Cora described as a “significant” right quad injury. Bregman suffered the injury during Friday’s game against the Baltimore Orioles. Mayer is batting sixth and manning third base in the second game of Saturday’s doubleheader.

Mayer, 22, opened the season in Triple-A with a 43-game stretch that saw him hit .271/.347/.471 with nine home runs and two stolen bases. He entered the spring ranked by CBS Sports as the 10th best prospect in the minors. Here’s what we wrote at the time:

Unavailability has become the theme of Mayer’s professional career. He’s still yet to play in 100 games in a season, having missed most of the second half of 2024 on account of a lumbar strain that prevented him from making his Triple-A debut. Mayer had a good year when he was hearty and hale, nearly matching Roman Anthony’s OPS at Double-A. He’s added strength (and swing and miss) since draft night, and he continues to be an effective shortstop with more quickness than straight-line speed. That makes for a good prospect, albeit one with a spotty health track record.

The Red Sox notably had three of CBS Sports’ top 10 prospects, including Mayer and fellow infielder Kristian Campbell, both of whom are now part of Boston’s active roster. Outfielder Roman Anthony, the No. 1-ranked prospect coming into the year, is also expected to debut over the course of the summer (though the Red Sox continue to resist using him to fill in at first base).

Below, CBS Sports has highlighted three things worth knowing about Mayer as he embarks on his initial tour of the big leagues.

1. Intriguing lefty hitter with power

Mayer has always drawn praise for his offensive potential.

As noted above, he’s added muscle since his amateur days, a development that has allowed him to clear 95 mph this season on more than 48% of his batted balls. His 111.2 mph maximum exit velocity (established in mid-April) puts him on roughly the same footing as Wilyer Abreu and Rafael Devers, or the top two left-handed sticks in the Red Sox’s lineup. There’s legitimate thump here, in other words, even if it’s primarily coming to his pull side.

That power will ultimately be the key to Mayer’s production, and it will have to atone for some negative aspects of his profile.

He’s always been a better hitter against righties than lefties, with the latter doing a better job of getting him to chase and whiff. He’s also had issues with offspeed pitches. In Triple-A, he whiffed on more than 40% of the swings he took against changeup and more than 34% of those taken against sweepers and sliders; the MLB averages for lefty batters are 29.5 and 31.7% respectively.

2. Could be key to infield realignment

While Mayer is a shortstop by trade, he had been receiving playing time at other positions in an attempt to improve his optionality.  Overall this season, he had received 28 starts at shortstop, 10 at second base (including four in a row to close out his time at Triple-A), and four at third base.

It’s to be seen how, precisely, the Red Sox deploy Mayer at the big-league level. The path of least resistance would have him slotting in at the hot corner. That would enable Campbell and Trevor Story to continue to form Boston’s most-days double-play combination.

At the same time, it’s likely that the Red Sox had a different plan in mind when they gave Mayer so many recent, repeated starts at the keystone. It’s possible that the Red Sox use him there and have Campbell play somewhere else — be it in the outfield or first base, where he’s continued to work out before games.

The Red Sox have been relying upon Nick Sogard and Abraham Toro at the cold corner as of late since losing both Triston Casas and Romy González to the IL.

3. Part of 2021 draft

Mayer was, naturally, Boston’s first pick and the crown jewel of their 2021 draft. Yet that particular class has already paid some smaller dividends ahead of his arrival.

Indeed, Mayer will become the fourth member of the Red Sox’s draft class to reach The Show, joining starting pitcher Hunter Dobbins (currently part of the starting rotation); outfielder Niko Kavadas (traded to the Los Angeles Angels last summer as part of the deal for reliever Luis García); and right-handed reliever Luis Guerrero.

In addition to trying to make the 2021 draft a win, Mayer will be attempting to become the first Boston first-round infielder to amass at least 5 Wins Above Replacement since Jed Lowrie (2005).