The Mariners added another name to their bullpen mix on Monday, claiming right-hander Isaac Mattson off waivers from the Pittsburgh Pirates.
It’s the kind of move Seattle tends to make this time of year: low-risk, practical, and aimed at building out options before Spring Training competition gets underway. Mattson gives the club another arm to evaluate, and his track record says there’s at least something worth examining.
The encouraging version of Mattson is easy to find. In 2025, he posted a 2.45 ERA and 1.13 WHIP over 44 appearances for Pittsburgh, a stretch that showed he can handle meaningful major-league relief work. Seattle isn’t trying to manufacture a pitcher out of thin air here; he’s already done it before.
The tougher part is that his most recent season was a lot messier. Mattson finished 2026 with a 5.35 ERA in 38 appearances, giving up 18 walks while striking out 36 across 38 2/3 innings. His 1.60 WHIP reflected the traffic he allowed, and that’s the version of him the Mariners have to sort through first.
His pitch mix is part of the puzzle. Mattson averaged 94.2 mph on his four-seam fastball in 2026 and used it at least 70 percent of the time in each of the past three seasons. He also throws a sweeper, slider and changeup.
That fastball usage is the obvious place for Seattle to start. A low-velocity four-seamer that dominates the mix puts a lot of pressure on command and execution, and the Mariners will need to figure out whether there’s a better version of him in there through improved location, a different mix or some other adjustment.
For now, this is less about a finished product than another bullpen candidate. Seattle has made a habit of collecting arms, and Mattson fits the profile of a pitcher with a recent strong season, a rough follow-up, and enough major-league work to justify a real look. If he finds the 2025 version again, the Mariners may have picked up useful innings from a pitcher Pittsburgh moved on from.
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