What Jack Flaherty Has in Common with Clayton Kershaw

Cardinals righty Jack Flaherty didn’t have what you’d call a “flawless” introduction to the majors. While he had some luck missing bats over his roughly 20 innings, he allowed too many walks and really struggled once batters made contact. His 6.33 ERA was 50% worse than league average after accounting for park and league.

The 22-year-old did, however, do one thing right: he threw 87 excellent sliders. The sliders were so excellent, in fact, that Flaherty recorded better numbers on the pitch than anyone else in the second half — just better than the ones thrown by Clayton Kershaw and Garrett Richards. Maybe he can learn something from those other two and help parlay his excellent slider into more excellent outcomes.

There are a lot of ways to judge the quality of a pitch. Some may be more sophisticated, and perhaps better, than the way I’ll use here. I’m not sure which is the exact correct way, though, so I’ll use a simple method: two times the relative quality of the whiff rate, plus one times the relative quality of the ground-ball rate.

By doing things so simply, I hope to avoid looking too hard at small buckets, or giving too much weight to a fly ball that turned into a double by way of a poor outfield route. I used twice the whiff rate because, when I correlated pitch-type whiff and grounder rates to overall results, I found that whiff rate was twice as strongly related.

Here are your best sliders thrown by starting pitchers in the second half last year by this estimation!

Only 10 sliders got more whiffs than Flaherty’s in all of baseball, and they were all thrown by relievers. (Pedro Strop was once again No. 1 with 33.7%.) Though you could argue that the slider isn’t used for ground balls, the pitch type is put in play like any other, and once it is in play, keeping the angle low is a benefit. Only eight starters induced more grounders with their sliders.

Let’s look at the pitch, because something should immediately become clear when you’re looking at the top of this leaderboard.

That’s a typical slider from Flaherty. Now here’s one that’s representative of the sort Garrett Richards normally throws.

Clayton Kershaw’s slider changed a bit over the course of the season, but here’s one that looks like his overall average.

This is why pitching is so hard to study: each of these sliders is objectively good and just as objectively different.

Still, the other starters on this list are almost all excellent — or, at the very least, effective starters. Flaherty may join the ranks with a larger sample, but it is still interesting to compare his overall arsenal to those possessed by the other two at top of this list — mostly because neither Richards nor Kershaw ever really throw a changeup. And Flaherty’s changeup was responsible for half of the home runs he gave up last year, even though he threw it just 6% of the time.

It’s not a good changeup, probably. Take a look at Flaherty’s pitch percentiles, for example. The change stands out as being a problem in all of the dimensions that are usually important for that pitch.

It’s sort of amazing that the best slider by results doesn’t look so great by movement or velocity, but if you follow the links above, you’ll find that the research on sliders is not as far along as it is on curves and changeups and fastballs. Give Flaherty the slider his scouting reports and results say he has, and he’s got a straight four-seam with good velocity, a great slider, and what looks like it could be a very good curveball.

Now, about that curve. Flaherty’s curve fares well by velocity and movement and spin. It gave up a .120 isolated slugging percentage last year. It was put in play eight times, leading to just a single and a double. It elicited an above-average whiff rate and an average ground-ball rate.

Flaherty’s changeup, meanwhile, rates poorly by velocity and movement defined off the fastball. It gave up a whopping 2.000 ISO last year. It was put in play three times, twice for homers. It had a poor whiff rate (4% against 14% on average) and didn’t even get grounders half the time.

Clayton Kershaw and Garrett Richards might be better pitchers than Jack Flaherty, but at least the Cardinal has a slider that can hang with theirs. Maybe he’ll take step towards their kinds of results if he follows their lead and jettisons his iffy changeup.