When evaluating starting pitchers, one of the things scouts and front offices look for is the number and quality of pitch types thrown. Most commonly you want a four-pitch pitcher, which often manifests itself with the standard pitches: 4-seamer, slider, curve, and change-up. Many pitchers have five or sometimes even six pitches, while it's also possible for starters to survive on just three. For example, Clayton Kershaw, best pitcher on the planet, just spun a 2.13 ERA and 1.99 FIP over 232 2/3 innings while using a fastball, slider, and curve 99.5% of the time this past season.
Tyler Duffey technically has three pitches. He might actually have four, to be honest. But for the minds that really matter, he really only throws two. You see, Duffey relies on a 4-seam fastball, that wicked curve, and a 2-seamer. No slider, no change-up, nothing else. His fastball does have a little bit of cut sometimes, but I reviewed our pitching notes from Inside Edge and we were unable to identify any called cutters for Duffey. Therefore, the movement is either accidental or catchers have Duffey choose when to throw it straight and when to cut it, which does happen for some pitchers.
As you can see in the pitchF/X graph above, the three main pitches are very clear. There is a little cluster in the lower right of the 4-seam fastballs that could be the cutter, but F/X doesn't see enough variation to call it a separate pitch.
We know that Duffey was the best starter for the Twins this past season, but what he did was unusual. No, not because he was a rookie or because I'm making a veiled joke at the Twins and starting pitching. Rather, I'm talking about the rarity of a pitcher using just fastballs and a breaking pitch to succeed in the rotation.
When you're a reliever, you don't need a lot of pitches. You're not supposed to be facing hitters more than once so you can get away with a limited repertoire. However, a starting pitcher is going through the batting order three, maybe four times, so he needs to have a mix of pitches to lean on to give the batters different looks each time.
Now, Duffey's curveball is so devastating that he used it 40% of the time last year. That's insane as Tyson Ross' slider was the only breaking pitch used more often by a qualified starting pitcher last season. If you reduce the innings restriction to just 50 and ignore all relievers, the only other pitcher ahead of Duffey is Chris Young and his slider. So, Duffey is certainly in unusual territory here.
I know that he looked so, so good last season, but remember that we can't just count on 58 innings to determine a player is a godsend. Many of us did this with Andrew Albers a few years ago, and all he's done since 2013 was throw 2 2/3 innings in the majors. Granted, that's partially because he was in Korea for a year, but I think the fact that he left MLB supports my argument rather than hurts it.
I get the feeling that if Duffey wants sustained success in the major leagues (and who wouldn't?) he needs to develop a third pitch. I do understand he has that 2-seamer, but again, that's a pitch that is virtually the same as his 4-seamer, even if it does have less "rise" than Kyle Gibson's sinker. If you go back to the F/X graph above, Duffey did tinker with a change-up last season, but it was thrown just over 2% of the time. Basically, it was a show-me pitch that he was never comfortable using.
Last season in his 58 major league innings, Duffey was actually stronger against lefties (.664 OPS) than righties (.738 OPS). However, that was unusual for him as he typically neutralized righties in the minors (.582 OPS) while lefties were more human (.733 OPS). Although the curveball does exhibit a reverse-platoon split (it's more effective against opposite-handed hitters), Duffey's curveball has a ton of sweep to it which makes it more similar to a slider, a pitch that has a large platoon split (it's more effective against same-sided hitters). Therefore, polishing another pitch that takes out opposite-handed hitters would be imperative for Duffey to succeed in future seasons as a Twin. What pitch also features a reverse-platoon split? The change-up, the very pitch he threw only 23 times last season.
When Duffey arrives at spring training in February, I'm sure pitching coach Neil Allen will have the change-up be Duffey's primary focus. Developing that pitch is likely the final puzzle piece needed if Duffey wants to remain a starting pitcher for the rest of his career.