Mariners claimed RHP Isaac Mattson off waivers from the Pirates

Mattson struggled in 2026 with a 5.35 ERA in 38 2/3 innings out of the bullpen, but the 31-year-old was much better for the Pirates in 2025, posting a 2.45 ERA, 1.13 WHIP, and 45/19 K/BB ratio in 47 2/3 innings. The Mariners will try to help him get back to that level of production.