Mariners Claim Reliever Isaac Mattson From Pirates

Mattson joins Seattle after a difficult 2026 season: in 38 appearances, he posted a 5.35 ERA and a 1.60 WHIP, with 36 strikeouts and 18 walks over 38 2/3 innings. His results fell well short of his 2025 performance, when he recorded a 2.45 ERA and a 1.13 WHIP in 44 appearances for Pittsburgh.

That earlier season gives the Mariners reason to consider what Mattson could offer, but the claim does not guarantee him a place in the bullpen. He will compete with other pitchers for a role as Seattle builds options for the coming season.

Mattson’s fastball averaged 94.2 mph in 2026, and he relied on it for at least 70 percent of his pitches in each of the past three seasons. His other offerings include a sweeper, slider and changeup. Seattle will assess whether adjustments to his execution or pitch mix can help him regain his earlier effectiveness.

The Mariners are adding a candidate with a record of major-league success as well as a recent season that raises questions. His spring competition will determine whether he can provide useful relief innings.