Just because the Cubs refuse to invest in expensive starting pitching doesn’t mean Stewart isn’t an interesting pick-up in a vacuum. In 2017, FanGraphs respectively graded Stewart’s fastball, slider, and curve as 60, 55, and 55 on the 20-80 scouting scale. Statcast data from his brief 62-inning MLB career appears to back up scouts’ initial grades.
Specifically, let’s compare his movement to that of all right-handed MLB pitchers. What you see below in bright red is how the horizontal or vertical break of his four-seam, two-seam, change, and curve stack up in terms of percentiles. Stewart’s sinker, thrown once every other pitch, has more dropping action than 84% of all sinkers thrown since data started being tracked. Ditto for his fastball.
But what’s extremely interesting is Stewart’s curveball, a tight-breaking pitch that sweeps towards the lefty batter’s box with more movement than 93% of all other curves. Think about that for a second. It passes the eye test, too.
While Stewart’s curve is his bread and butter, don’t think that changeup isn’t interesting. You’ll see in the table above that, similar to his fastballs, his change sinks with 90% more movement than his MLB colleagues. That change made Juan Soto look silly.
Okay, but if all of those pitches are so great, why hasn’t the 26-year-old former top draft pick had success in the bigs yet? Why are his strikeout numbers so low?
Remember, Stewart throws 50% sinkers and he’s pitching to contact. At a time when pitchers across the league are doing away with the sinker, the Cubs are taking a different tack. Pitching coach Tommy Hottovy and VP of pitching Craig Breslow are embracing the sinker differently by coaching their pitchers to throw it up in the strike zone.
Rather than throwing sinkers “the Cubs way,” Stewart has located almost all of his down in the zone. That’s why his grounder rate hovered around 55% throughout his career, but it’s also prevented him from missing many bats and has decreased his overall margin for error.
Just below, you’ll see sinker heat maps for Kyle Hendricks, José Quintana, Adbert Alzolay, and Alec Mills from 2020. Notice how there are red spots and waves above the midpoint of the zone. Brailyn Márquez just developed a new sinker at the advice of the Cubs pitching infrastructure as well, so perhaps we’ll get a chance to see where he locates it if and when he’s back up this season.
I’m trying to imagine how Stewart can be successful by making a few adjustments to his location and perhaps his sequencing. Think about a flurry of sinkers thrown high in the zone with wicked offspeed and breaking stuff to complement them. He could elevate to get ahead 0-2 quickly and then, boom, he uses that wipeout curve to strike batters out.
Looking at Stewart’s historical performance probably doesn’t give us an accurate picture of what kind of pitcher he can really be for the Cubs. If he’s able to better leverage what appears to be really good stuff, he might make that $700,000 contract look like an incredible bargain.