Mariners Bet on Isaac Mattson’s Bounce-Back Potential With Pirates Waiver Claim
In this story:
Isaac Mattson has already shown he can get big-league hitters out. The Mariners just volunteered to see if they can resurrect that version. They claimed the righty off waivers from the Pittsburgh Pirates on Monday, adding another arm to its offseason bullpen competition. It’s a sensible pickup with a straightforward appeal. And it’s never a surprise to see the Mariners stock up on bullpen arms to prepare for competition during Spring Training.
A bullpen needs options, and November is a good time to collect them. Mattson gives Seattle something to look into before the games start counting.
His most recent work also gives us plenty of reason to temper expectations. He finished 2026 with a 5.35 ERA in 38 appearances, allowing 18 walks while striking out 36 over 38 2/3 innings. His 1.60 WHIP tells us traffic was a regular feature of his outings. That followed a much stronger 2025, when he delivered a 2.45 ERA and 1.13 WHIP across 44 appearances for Pittsburgh.
Isaac Mattson Gives the Mariners a Worthwhile Bullpen Project
The 2025 season matters because it shows us that he can be a productive guy out of the pen. Mattson has handled a meaningful stretch of major-league relief work successfully. The Mariners aren’t trying to make something out of nothing here.
A good season buys an opportunity. But it doesn’t guarantee him a bullpen spot. The Mariners should want a serious competition for those jobs.. Every additional pitcher capable of earning innings gives the Mariners a better chance of avoiding the familiar summer scramble, when a tired bullpen is gassed and can’t close out close games.
Mattson’s pitch usage offers one place to start the conversation. He averaged 94.2 mph on his four-seam fastball in 2026 and threw the pitch at least 70 percent of the time in each of the past three seasons. He also has a sweeper, slider and changeup.
Honestly, the fastball sounds concerning. It should be an obvious priority for Seattle’s eval. Using a low velocity four seamer 70 percent of the time is rarely going to give you great results. When a pitcher leans that heavily on one offering, its location and effectiveness carry a lot of the workload. The Mariners will need to determine whether better execution, a different mix or another adjustment can help him recover.
There’s room to appreciate this move without attaching a comeback story to it. Bullpen depth becomes useful when someone can actually take the ball and protect a game. Until then, it’s a list of candidates. Mattson joins that list with a recent success worth revisiting and a rough season that needs explaining.
The Mariners should give him a real chance to compete, hold him to a clear standard and keep looking for pitching that makes the bullpen stronger. For now, we have a reasonable bet. If Mattson rediscovers his effectiveness, the Mariners will have found useful innings in a player Pittsburgh let go.
Add us as a preferred source on Google
Tremayne Person is the Publisher for Mariners On SI and the Site Expert at Friars on Base, with additional bylines across FanSided’s MLB division. He founded the Keep It Electric podcast in 2023 and covers baseball with a blend of analysis, context, and a little well-timed side-eye just to keep things honest. Tremayne grew up a Mariners fan in Richmond, Va., and that passion ultimately led him to move to Seattle to cover the team closely and become a regular at home games. Through his writing, he connects with fans who want a deeper, more personal understanding of the game. When he’s not at T-Mobile Park, he’s with his dog, gaming, or finding the next storyline worth digging into.
Follow TremaynePerson