Mariners demote infuriating reliever Gregory Santos in favor of intriguing prospect

The Seattle Mariners have officially seen enough of Gregory Santos, as the team announced on Wednesday that he has been demoted to Triple-A Tacoma.

The team revealed the news via X, also noting that right-hander Will Klein has been recalled to take Santos' place in the bullpen. Klein was already a member of the 40-man roster.

The demotion marks yet another frustrating chapter for Santos' time in Seattle. Originally acquired to work as a relief ace alongside Andrés Muñoz, he has instead battled injuries and done more harm than good when he has been able to pitch.

The Mariners hope that Will Klein can do what Gregory Santos couldn't by living up to his upside

The Mariners' initial faith in Santos was well-founded. Before coming over from the Chicago White Sox in February, 2024 trade, he had broken out in a 2023 season that saw him post a 3.39 ERA while landing in the top 10 percent of pitchers in fastball velocity, chase rate, and barrel rate.

Eventually, the 25-year-old made only eight appearances in 2024 as a result of lat and biceps injuries. The Mariners could hope that better things were on the horizon for 2025, but Santos wasted no time in testing their patience to its breaking point.

In eight appearances, the righty permitted six runs (four earned) on eight hits and eight walks. He struck out nobody, with his final humiliation — at least for now — being an appearance against the Cincinnati Reds on Tuesday in which he got only one out while otherwise allowing two runs on a hit and three walks. It was all part of a frustrating 8-4 loss that dropped the Mariners to 8-9 for the year.

For his part, Klein is a fellow 25-year-old who has yet to exhaust his rookie eligibility, hence why we're comfortable calling him a "prospect." He has a 12.79 ERA in seven appearances for Tacoma in 2025, but that figure is misleading.

Klein has had two very bad outings, but has otherwise been solid and missed his share of bats. He's fanned 13 of the 36 batters he's faced, showcasing a fastball that has gotten up to 99 mph and a curveball with a 50.0 whiff percentage.

With Troy Taylor already back and Matt Brash not far behind, Klein could be in for a short stay with Seattle's bullpen. He has two minor league options left, however, so the M's could always bring him back later in the season if need be.

For now, at least, it's hard to imagine Klein being any less reliable than the guy he's replacing.