Isaac Mattson came out of nowhere for the Pittsburgh Pirates’ bullpen in 2025. Prior to ‘25, his career consisted of minor league depth and a few stints in independent league ball, including with the Washington Wild Things. In 47.2 innings, the right-hander posted a 2.45 ERA, 3.66 FIP, and 1.13 WHIP. His 22.7% K% was around league average. He also had a 9.6% walk rate. Mattson excelled at limiting home runs and quality contact, with just a 0.76 HR/9 ratio and 6.1% barrel rate.
https://www.youtube.com/watch?v=Rg0RmOloRyU
By the end of the year, Mattson became a high-leverage option for the Pirates. His average leverage index was 1.77 from August through the end of the year. However, Mattson didn’t start 2026 off on the right foot. He allowed nine earned runs, including ten walks, with only 15 strikeouts throughout his first 19.2 innings of the season. His ice-cold start led to the Pirates demoting Mattson to Triple-A in the middle of May.
The main culprit of Mattson’s struggles stems from his fastball. Although the pitch didn’t change much in terms of velocity or movement, opponents were much less fooled by it. Last year, Mattson held opposing batters to a .288 xwOBA with a 23.1% whiff rate with the pitch. This year, he has a .381 xwOBA with just a 17.8% whiff rate. The problem is that his fastball is his most-used offering by far. In 2025, he used his four-seamer 78.5% of the time, and was using it 80.7% of the time this year.
While Mattson’s four-seamer has above-average movement, and his plus extension makes the pitch play up, as he only sits 93-94 MPH. Given that he threw just a fastball over three-quarters of the time, Mattson became a lot more hittable as batters sat on the fastball. However, Mattson is trying to correct that. Since his demotion, he has worked on incorporating more into his pitch arsenal.
Mattson’s fastball is still an integral part of his pitch arsenal, but its usage has been lowered to 55.3%. His most-used secondary pitch is a slider, which he has thrown 23.7% of the time. He has also used a reworked curveball 11.8% of the time, a pitch he threw just 9 times in 2025 and 2 times in 2026 before his demotion. The only pitch in his arsenal that hasn’t seen a major change in usage is his change-up, which he has utilized 9.2% of the time at Indy, compared to 9.2% of the time before his demotion, and 11.6% of the time in 2025.
For what it’s worth, Mattson has posted respectable results at Triple-A. He has only allowed two earned runs over 5.2 innings. He is keeping batters off balance. His whiff rate throughout this small sample size of play is 43.9%. His fastball (50%), slider (36.4%), and curveball (50%) have induced a whiff on more than a third of swings.
It was clear that Mattson needed to integrate more pitches into his arsenal. Even relievers with elite heaters like Mason Miller, Jhoan Duran, and Aroldis Chapman have at least two pitches they use 20%+ of the time. Hopefully, his reworked pitch mix will put him back on track to becoming an effective reliever. With the Pirates needing relief help badly, they should see if Mattson’s reworked pitch mix will help him succeed in the Major Leagues in 2026.
