Isaac Mattson brings intriguing fastball to Mariners after 40-man roster shuffle

The Seattle Mariners had to trim their 40-man roster down to an actual grouping of 40 players on Monday, and they did so while adding a new face to the mix: Isaac Mattson.

As reported by MLB Trade Rumors, the Mariners added the 31-year-old right-hander from the Pittsburgh Pirates via a waiver claim. He's joining the roster with 89 major league appearances under his belt and, crucially, one minor league option remaining.

Mattson had his big break for the Bucs last year, appearing in 44 games and pitching to a 2.45 ERA. Opposing batters hit just .191 against his four-seam fastball, which he threw close to 80 percent of the time. It only averaged 93.9 mph, but his extension was in the 77th percentile and the pitch had 2.9 more inches of induced vertical break than the average fastball.

Despite an increase up to 94.2 mph with his average velocity on the heater, Mattson was unable to carry his 2025 success over to 2026. His 38 appearances with Pittsburgh yielded a 5.35 ERA, in part driven by a .259 average against his fastball, which lost some of the vertical break it had featured in the prior season. He also walked 18 batters in just 38.2 innings, signaling the return of control problems that have plagued him throughout his professional career.

Yet when it comes down to it, this is the time of year for teams to stack depth. Mattson checks a box simply in that respect. Should he one day find himself in the bullpen, that fastball of his would at least promise a different look from the fastball-slider mixes of the incumbent righties.

Mariners' 40-man roster is down to 39 after outrighting five other players

When the claim on Mattson went through, the Mariners had 44 men on their 40-man roster and a 1:00 p.m. PT deadline to cut it down to size. Ultimately, they outrighted five players to Triple-A Tacoma:

• LHP Josh Simpson
• RHP Troy Watson
• INF Weston Wilson
• INF Will Wilson
• 1B/3B Patrick Wisdom

Wisdom is the most notable name on this list. He's a veteran of eight major league seasons, and the Mariners added him when he was fresh off a 35-homer season in Korea for the Kia Tigers. He also hit 35 homers in just 81 games for the Rainiers this season, but only one in 19 games for the big club.

Wisdom, 35, had been eligible for arbitration with a projected 2027 salary of $1.1 million. The Mariners figured to non-tender him even if they hadn't outrighted him. Since he has more than four years of MLB service time, Wisdom also has the right to elect free agency in lieu of accepting the assignment to Tacoma. If he's with the Mariners again in 2027, it may be because the two sides hammered out a new minor league contract.

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