The Stuff That Dreams Are Made Of: Zach Stewart
By Aaron Somers
Many popular opinions of pitching prospects are formed from general scouting reports. While these reports are invaluable resources, they can’t always be trusted. Hundreds of minor league hurlers are credited with “mid-90′s velocity,” but very few MLB starters actually have that grade of heat, for example. It’s incredibly frustrating to hear about a pitcher with “a mid-90′s heater and plus curve,” only to have him come up to the big leagues and show a fastball that averages 90.5 mph and a slider.
When a pitcher come up to the majors, we can finally get a foolproof reading on what exactly his arsenal is comprised of, thanks to the great Pitch F/X system. In this series, I analyze just that–the “stuff” of recently-promoted MLB pitchers. Now that they’ve achieved their big league dreams and thus factor directly into the MLB picture, it’s high time that we know exactly what these guys are providing.
This time, I’m taking a look at White Sox starter Zach Stewart.
I’m going to say this up front: I always found Zach Stewart to be an overrated prospect in the minors. He never put up the sort of strikeout numbers to show that he’d be anything more than a middle reliever or back-of-the-rotation starter, yet many evaluators were quick to heap praise on the young righthander.
But, hey, here he is, with 63 1/3 reasonably solid big league innings (4.01 FIP) as a 24-year-old rookie (he turns 25 on the last day of the season), so perhaps he can outperform my expectations and meet those of others.
Like Marlins starter Brad Hand, who I profiled yesterday, Stewart throws a fastball that fails to approach the velocity he was often credited for in the minors. Baseball America said he “routinely touches 95-96 mph” before the season, but his fastball has averaged just 90.7 mph in his first big league season. That’s a significant difference in scouting, and can easily change the perception of a pitcher.
Stewart’s fastball does have solid movement, and he has a low-maintenance delivery that allows him to spot the pitch easily. That, combined with his tendency to pound the zone with the pitch, explain his walk rate of 2.56 BB/9. Here’s a look at where he puts the fastball to righties:
And lefties:
A couple of things jump out here. On the positive side, Stewart clearly looks to pound the lower part of the zone to righties and the outside corner to lefties. The bad news is that there are quite a few pitches left out over the plate to both, and Stewart doesn’t really have the velocity to get away with that. As a result, he’s given up an above-average number of line drives; he’s also allowed more homers than most groundball pitchers.
Stewart pairs the fastball with a hard breaking ball that he varies the speed on. Pitch F/X calls it a slider that he throws anywhere from 75-85 mph, whereas Baseball Info Solutions’ data (the pitch type classifications on FanGraphs) classifies about 3/4 of the pitches as sliders and the other 1/4 as curveballs.
The “curveball” looks to be more or less a slower, bigger version of the slider, and since the raw Pitch F/X data calls it all one pitch, I’ll treat it that way here for simplicity’s sake (and because that’s the data I can get advanced stats and graphical views on!). It’s a pretty nasty pitch, showcasing hard, late break. Against righties, he throws an awful lot of sliders in or just above the dirt:
Against lefties, Stewart either backdoors the pitch or throws it as a chase pitch. That does eliminate the issue of leaving the ball in the middle of the plate, but also means he gets strikes just 54% of the time with it to opposite-side batters:
Since he doesn’t get many strikes with the breaking ball to lefties, and he doesn’t get that many whiffs on it (just a 7.3% whiff rate), he doesn’t have too much that he can do to them other than work away-away-away with the fastball and hope the hitters get themselves out. After all, he barely has a changeup–he uses it under 5% of the time. It’s about seven mph slower than his fastball and features some movement, and it’s actually been effective when he’s used it, although that can be chalked up to both the very small sample and the fact that hitters are never looking for it since he throws it so rarely.
Add it all up, and you have a pitcher who can get righthanders out very well (4.80 K/BB), but is relatively helpless against lefties (1.54). Stewart is exactly the sort of pitcher who could benefit from adding a cutter, to give him something to work the inside part of the plate to lefties, because right now he has nothing he can throw inside to them, making him predictable on top of lacking a pitch that can get them to swing and miss.
He’s so efficient with righthanders, however, that he can stick in the rotation. There are clear flaws here, and unless he develops the changeup a lot or adds a solid cutter, it’s hard to see what Stewart can do to make substantial gains. That leaves him as the sort of pitcher that can eat innings as a fourth starter, in the mold of guys like Kyle Lohse, Wade Davis, or Jeremy Guthrie. That’s probably less than what most people expected, but it’s more than I did, and Stewart should have one of the Sox’s five rotation spots locked up for the next half-decade.
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