BRADENTON, Fla. — Isaac Mattson has heard his disappearing four-seam fastball described as a unicorn fastball, invisible fastball or even the invisi-ball, but the Pittsburgh Pirates right-handed reliever has his own name for it.
“I just call it a heater,” Mattson said with a smile.
It’s a heater delivered with deception that has developed into a devastating offering because of its effectiveness. The pitch was almost unhittable in 2024, when opponents batted .083 against Mattson’s four-seamer. They fared only slightly better (.191) last season, when he used it to record 35 of his 45 strikeouts.
For Mattson, this sword-serving pitch is double-edged.
“There’s the one side where I’m doing something cool and I know that this can be a strength for me and I can get guys out with just one pitch, if that’s what it needs to be,” said Mattson, 30. “But the flip side is, you kind of pigeonhole yourself. If you get yourself to bad spots, then guys start to key on it.”
That’s why Mattson is spending spring training working with new pitching coach Bill Murphy and Pirates catchers to balance his arsenal with complementary pitches. Where Mattson threw his four-seamer on 79% of his pitches, he used the changeup on only 12% and the slider on 9%, even though hitters batted only .167 against the latter.
Mattson is working to tunnel the slider effectively against right-handed hitters, given that it looks like his heater for the longest amount of time.
“That’s been the goal this spring training,” Mattson said, “selling heater arm speed and being able to throw that to good locations again to not trick the hitter but give him a look where it’s not something early out of the hand like a curveball where you’re seeing the pop-up. It’s on the same line as the heater, and, at the last second, it dives away.”
Mattson also wants to do better with his changeup against lefties, who batted .267/.389/.440 against him last season with 18 strikeouts and 14 walks. By comparison, Mattson was dominant against righties, who batted .155/.198/.216 with 27 strikeouts against five walks despite an average velocity of 93.9 mph that is a tick below the league average
“He’s evolving, being able to throw the changeup and spinning it a little bit, too,” Pirates manager Don Kelly said. “That fastball is just elite, just the way he hides it. There’s deception there. It’s amazing he can, even in the low 90s, blow it by guys. Then when he gets it up to the mid-90s, it’s a really effective pitch.
“He’s a traditional guy that pitches off the fastball. If he can expand and throw that changeup and breaking ball, it would really make him tough to hit.”
Mattson has shown just how tough he can be to hit in Grapefruit League play this spring, recording four strikeouts without a walk and one hit allowed in four scoreless innings. He earned the win Monday night in a 5-3 victory over the New York Yankees by retiring all four batters he faced.
Mattson replaced Jose Urquidy with two outs in the bottom of the fourth inning and threw four consecutive four-seamers to Oswaldo Cabrera, getting him to fly out to center on a 94.5-mph heater. In the fifth, Mattson started with three fastballs against Paul DeJong before mixing it up by throwing a slider, changeup and fastball to work a full count before using a slider at the top of the strike zone to get a fly out to left field. Mattson threw five straight fastballs to Payton Henry, getting him swinging at one on the outside corner. Mattson then threw a changeup to Trent Grisham to get a groundout to second base.
Mattson believes his commitment to the fastball — no matter what you call it — comes down to conviction. He’s hoping it translates to his off-speed and breaking pitches but also knows to use what works best.
“When I came to the Pirates, there was a big stress on using strengths,” Mattson said. “Coming into it knowing that’s what they wanted me to do — obviously, there’s other aspects of pitching, where we’re working on off-speed and trying to get that as good as it can be — but when it comes down to winning baseball games and executing at high levels, they really want guys to use their strengths and use them effectively.”
