The Pittsburgh Pirates didn’t take a collective step forward this year, but one player who surprisingly did was reliever Isaac Mattson.
A local kid who went to the University of Pittsburgh before becoming a 19th-round draft pick, Mattson wasn’t much of a prospect. He pitched sparingly and entered 2025 with a 5.59 ERA in 9 2/3 career innings, so he wasn’t expected to be a big contributor.
Mattson started the season at Triple-A Indianapolis, but something clicked for him down there. In 19 appearances for the Indians, he went 2-0 with a 2.57 ERA, getting his confidence back and earning a call-up to the Pirates in late May.
Shockingly, the 30-year-old rookie proved to be one of Pittsburgh’s best relievers this year, going 3-3 with a 2.45 ERA, a 1.13 WHIP and 1.5 WAR in his first full season.
Mattson doesn’t have overpowering stuff, averaging fewer than a strikeout per inning this season. However, he dramatically improved his control this season, lowering his walk rate from 21.7 percent prior to 2025 to 9.6 percent.
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Using a simplified approach, Mattson focused more on getting first-pitch strikes and staying ahead of batters, making it easier to fool them and get them out with his offspeed stuff.
The veteran righty faded a bit down the stretch, however, with a 4.22 ERA, a 5.29 FIP and more walks than strikeouts in September. Time will tell if that was merely fatigue or opposing batters catching on to him, but the Pirates are hoping he can be a key part of their bullpen again in 2026 and beyond.
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