The Evolving Curveball of Kyle Hendricks

As you’ve likely heard, the first World Series game at Wrigley Field in seven decades will be played this evening. The starting assignment belongs to Kyle Hendricks, the soft-tossing right-hander lovingly known as “The Professor.” At this point, Hendricks has done enough to convince the attentive fan that he’s an above-average major-league pitcher. While many of us were on board with Hendricks in 2014 and 2015, there might have still been cause to doubt a pitcher whose fastball sits at 88 mph. After a 2016 season during which he both maintained his strong fielding-independent numbers and allowed very few runs, there isn’t much room left for doubt.

Hendricks has further cemented that impression on the biggest stage, allowing just three runs in 16.1 innings this postseason to go along with his consistently strong fielding-independent resume. Even if you give plenty of credit to the Cubs’ superb defense for Hendricks’ top-line numbers, it’s hard to ignore his performance this season and over the last few weeks.

Earlier this year, I noted that one of the interesting things about Hendricks was his ability to get a significant number of ground balls while also generating a fair number of infield flies. While his strikeout and walk numbers suggest Hendricks is quite good, the fact that a lot of his balls in play are easy outs (infield flies) and unlikely to leave the park (ground balls) indicates that some of his contact management is his doing, independent of his great defense.

As you might have expected, in the 21 regular-season starts that took place after I wrote that piece, Hendricks’ batted-ball profile came back to Earth. His ground-ball rate fell right in line with league average and his pop-up rate regressed as well. Whenever you point out a pitcher that’s doing something exceptional, chances are some of it is buoyed by random variation; that appears to have been the case with Hendricks’ great batted-ball profile. The interesting thing, however, is that, while Hendricks’ balls in play started to look more typical, his performance didn’t suffer.

Between April and May, Hendricks recorded an ERA 29% better than league average, but from June forward it was 57% better than league average. His FIP rose from 32% better than league average to just 19% better than league average, however. His BABIP was nearly identical in both splits and the FIP increase can largely be attributed to a home-run rate over his first nine starts that was simply unsustainable for anyone. Lost in the fielding-independent regression is a big increase in strikeout rate, from 20.6% to 23.8%.

Those splits are largely arbitrary, so let’s take a rolling average from 2016:

One of the main reasons Hendricks’ strikeout rate has improved as the season as progressed is that he’s getting more swings and misses at pitches in the strike zone:

There are a lot of things that influence contact rate within the zone. There’s no single way to unpack a change like this, but one thing that sticks out to me from watching Hendricks is the evolution of his curveball this season.

Pitching, of course, is complicated. Attempting to evaluate the effectiveness of Hendricks’ curveball isn’t as simple as looking at whether he’s getting more swings and misses in the zone against that pitch. Each individual pitch exists in the context of the others thrown by any one pitcher. For the most part, the changeup is Hendricks’ whiff-inducing option. However, the mere presence of an improved curveball might increase the number of swinging strikes against his other pitches.

In fact, Hendricks seems to be using his curveball more frequently against left-handed hitters the second and third time through the order. According to Brooks Baseball, he’s thrown it 4.5% of the time the first time through against lefties, but then 13.1% and 16.9% of the time on his second and third trips through the order. Last year, the same splits were about 3%, 10%, and 10%, respectively. Hendricks has been better against lefties across the board this season, but he’s experienced a lesser times-through-the-order penalty against them this year, as well. This isn’t surprising, given that the evidence seems to suggests a wider arsenal can limit the times-through-the-order penalty.

Hendricks is known for his command. His ability to locate his sinker and changeup are what got him to the big leagues. Amid his excellent 2016 season and Cubs’ postseason run, however, he’s also managed to improve the quality of his curveball. It’s a little too early to say exactly how this has impacted his performance. There’s some evidence that it could be helping him get swinging strikes in the zone because batters have to look for another quality pitch type, especially with two strikes, but the curveball itself isn’t all of a sudden destroying hitters.

Hendricks was a very good major-league starter when his curveball had much less movement, but now that it is becoming a better developed offering, it’s potentially becoming more difficult for hitters to develop a familiarity with his pitches and approach. Stuff, location, and command all influence pitcher success. Hendricks is known for the latter two, but the fact that he’s adding to the first should trouble Cleveland hitters and the rest of the league.