Hiroki Kuroda cashed in $15 million last season for the New York Yankees, which was the second-most of any starter on the roster (save for C.C. Sabathia's monster $24 million salary). At the end of July, it seemed as though the 38-year-old righty would easily live up to his lofty billing, having posted a 2.38 ERA and 1.03 WHIP (sixth and tenth-best, respectively, among qualified starters up to that juncture) over 22 starts.
And then August happened.
Starting the month by allowing just three earned runs over his first two starts, Kuroda proceeded to allow 15 earned runs in his subsequent three starts (16.2 innings) to finish out the month. His struggles continued from that point on as he posted a 5.70 ERA over five September starts -- enough to raise his 2.38 ERA over the first four months of the season to 3.31, which was only slightly better than the 3.51 league average for qualified starters by the end of the regular season.
The reason for Kuroda's ERA spike? He stopped throwing in the strike zone.
Comparing Kuroda's Pitch Frequencies over both spans
To be fair, Kuroda has never been one to pound the strike zone at a high frequency. Since his rookie 2008 campaign, his zone% (pitches in the strike zone / total pitches) stands at just 44.7% -- including this past season -- which ranks as the seventh-lowest rate among qualified starters. This is over four percent lower than the 49% league average since 2008, and a whopping 13.8% lower than Cliff Lee, whose 58.5% zone rate since 2008 is easily the highest mark in baseball over that span.
Most of Kuroda's plate discipline numbers remained relatively steady over both spans last season. And in some cases, they actually improved -- his miss rate jumped to 23.5% from August to September and his chase followed a similar trend, increasing to 32.7%. The same cannot be said about opponents in-play average (increasing nearly .100 points) and home-run-to-fly-ball rate, which nearly doubled from August on.
From the beginning of last season to the end of July, 45.2% of Kuroda's pitches fell in the strike zone (somewhat close to the 49.8% league mark over that span). That number dropped to 38.1% from that point to the end of the season, which was the lowest mark of any qualified starter and a whole 5.3% lower than the guy with the next-lowest mark (Francisco Liriano's 43.4%). Consequently, Kuroda's zone% shrinkage directly affected opponents' increase in BABIP.
How he stacked up with the league...
...from April to July...
...and from August to September.
As we see, there seems to be a correlation between zone% and opponents BABIP last season -- when you throw less pitches in the strike zone, batters tend to have more success at finding holes in defenses. When Kuroda's zone% remained near his career average from April to July last season, his opponent BABIP (.255) was low. But when he deviated away from that mark by throwing only 38.1% of his offerings in the zone from August on, batters had much more success at finding holes in Joe Girardi's defense. Normally, you would think throwing more pitches in the zone would give batters a better opportunity to square up pitches and thus have a higher BABIP, but this clearly isn't the case.
My advice to Kuroda: Throw more pitches in the strike zone.