It's been a rough start to 2015 for the Klubot, but his last two outings may prove that he's back on the right track. The biggest question of his turnaround is just how did he do it? How did he go from looking like a potential one-hit wonder with his first few starts to being an 18-strikeout machine against the St. Louis Cardinals just a few weeks later? Thanks to the wonderful world of PITCHf/x we can get a pretty exact measurement on what changed.
The over-arcing answer is that Corey Kluber started to strike out more batters and walk less batters. In his last two starts against the Cardinals and the Chicago White Sox, Kluber struck out a combined 30 batters, and walked only 1. Prior to those starts however, he only managed to strikeout double digit opponents once against the Detroit Tigers in his second start of the season.
So, what changed specifically with Kluber to help put up his two amazing box scores recently? It comes down to two things: the kind of fastball he's thrown, and the amount of movement on all of his pitches.
Looking over his first few game charts, Kluber's pitches looked rather predictable. Pitch after pitch was the relative same amount of movement, and it wasn't much movement to begin with. Other than a few outliers, this narrative remains mostly the same until you get to his more recent starts where his pitches finally get the amount of movement that matches up with the Cy Young season last year.
Courtesy of FanGraphs
In particular we can see the movement on his slider and cut fastball greatly improving with his better starts, and peaking during his last start against the White Sox when he fanned 12. In his other good starts, notably that historic St. Louis game as well as the April 11th start against the Tigers, his change up has an added bit of vertical drop to it as well. Most importantly his sinker, the pitch that is most closely linked to emergence in recent years, also improves with each passing start.
Despite finding better movement on his sinkers in his last two outings, the biggest change is finding his groove with the four-seam fastball. According to Brooks Baseball, Kluber only threw four-seamers a total of 10 times over his first 6 starts. It wasn't until his start on May 7th start, where he went only 5.2 innings in a loss against the Kansas City Royals, when he started to lean on his four-seam fastball more. In that game alone he threw it 19 times. Despite the rough loss, Kluber went on to throw the four-seamer 26 times against the Cardinals and 28 against the White Sox to great effect.
This of course meant a big drop in his sink fastball usage which he threw an average of 48 times over those first 6 starts. In these most recent 3 games, he threw the pitch less than 35 times in each game - 29 against the Royals, 34 against the Cardinals, 33 against the White Sox.
With the decreased sinker usage his whiff percentage has increased dramatically with the pitch. Meaning when he does decide to use it, it is catching hitters off guard and/or blowing right by them. His whiff rate wasn't great in a lot of games to begin with, however. The pitch only fooled Houston Astros batters 2% of the time in his first start, 1.89% of the time against the Toronto Blue Jays, and not a single whiff came from the sinker in an outing against the Minnesota Twins. Against the Royals his sinker drew whiffs 6.90% of the time, and against the Cards it did so 11.76 of the time.
It will be interesting to see if this increased four-seam fastball usage comes into play as Kluber takes on the struggling Cincinnati Reds offense when the Tribe go for their third series win in a row this afternoon. If the recent trend should continue, the correct answer is obvious: he's going to throw it. A lot.