Mariners To Claim Isaac Mattson

The Mariners have claimed right-hander Isaac Mattson off waivers from the Pirates, MLBTR has learned. Pittsburgh hadn’t designated him for assignment, so it seems they instead quietly placed him on outright waivers at some point in the past couple days. Seattle’s 40-man roster is now four players over capacity, so they’ll need to subtract four players at some point prior to today’s deadline to get back down to 40.

Mattson, 31, has pitched in each of the past three seasons with the Pirates. He was a particularly solid arm for them in 2025, when he tallied a dozen holds and notched a spectacular 2.45 ERA with a 22.7% strikeout rate and 9.6% walk rate.

Mattson’s results went the wrong direction in 2026. Mattson was roughed up for a 5.35 ERA, although a weighty .347 average on balls in play and 68% strand rate were both well out of line with his career norms. Mattson did give up far too much hard contact (92.1 mph average exit velocity, 52.1% hard-hit rate), so the bloated ERA can’t be attributed solely to poor fortune on balls in play, but there’s still reason to forecast some positive/favorable regression for the righty.

In three seasons with the Bucs, Mattson logged a 3.83 ERA and 20 holds in 91 2/3 innings. He sits 94.2 mph on a four-seamer he uses at a nearly 75% clip, mixing in a relatively rare slider and changeup in a three-pitch repertoire. He also had a curveball early in his career, when he debuted with the 2021 Orioles, but he shelved that pitch back in 2024.

Mattson did tinker with his curveball a bit in Triple-A this year, throwing the pitch at a 5% clip. He was excellent in the minors, broadly speaking, as evidenced by a 1.33 ERA and 29.6% strikeout rate in 20 1/3 innings with the Pirates’ top affiliate in Indianapolis.

The 2027 season will be Mattson’s final minor league option year. He can be shuttled between Seattle and Tacoma throughout the season — if he sticks on the Mariners’ roster all winter, that is — but will be out of options beginning in 2028 (unless he spends the nearly entire big league season on the roster in Seattle next year). He reached one year of major league service in 2026, meaning the Mariners can control him for five more years if he makes their roster and carves out a spot in the ‘pen.