The San Francisco Giants have had a remarkably busy trade deadline day, and they just pulled off perhaps the most fascinating of all their moves. According to multiple reports, the Giants are sending left-handed reliever Erik Miller to the Boston Red Sox in exchange for infielder Marcelo Mayer.
Mayer is an incredibly intriguing pick up for the Giants. The 23-year old left-handed hitter was the fourth overall pick in the 2021 draft, and was largely viewed as a top-20 prospect in baseball before debuting.
But he’s struggled since his 2025 debut: in 364 Major League plate appearances, Mayer has hit just .223/.278/.346 with seven home runs and a 23.4% strikeout rate, good for a .625 OPS and a 69 wRC+.
Despite those poor offensive numbers, Mayer has still stayed afloat from a value standpoint thanks to his strong defense, as he’s been worth +0.3 fWAR throughout his short career, while logging playing time at second base, third base, and shortstop. The sample size is small, but so far in his career he’s graded out very well at second base, solidly at third, and fairly poorly at shortstop.
It seems likely that the Giants will give Mayer the full-time second base job with Luis Arráez traded and Casey Schmitt injured, and see what he can do over the next two months. They could also play him at third while Matt Chapman remains sidelined, so that Nate Furman can get some run at second base. That said, it’s unclear when he’ll be ready to join the Giants, as he’s currently on the 10-Day Injured List for a bone stress reaction, though he’ll be eligible to come off the IL in a day or two if he’s healthy.
The hope is that the Giants can help Mayer materialize some of his prospect potential. He hit comfortably above average in AAA last year, with a 113 wRC+ and a .200 isolated slugging percentage, and he was dominant in AA the year prior. The results and the batted ball data have been awful in the Majors, but he’s young and very inexperienced at the level. If his potential starts to shine through, the Giants will be in a great place: Mayer still has all three option years remaining, and has five more years of team control after this season. If he’s able to find his footing at the big league level, the Giants will have a cost-controlled core piece for the forseeable future.
Losing Miller hurts, though. The burly lefty had some of the strongest and nastiest stuff in the organization, and has for more years of team control after this season. San Francisco has a monumental task of fixing their awful bullpen for next year, and they just traded away one of the few bright spots from this year, a southpaw with a 2.76 ERA, a 3.34 FIP, a high-90s fastball, and 12.4 strikeouts per nine innings.
