Hiroki Kuroda was an undervalued free agent last year, and when the Yankees picked him up on a one year deal, a lot of us fans realized the type of potential he had. Even with high hopes on Kuroda, understanding how his 3.07 ERA would fare in the AL East and Yankee Stadium was a difficult task. Add his ERA/FIP differential and age into the analysis, and it was hard to analyze how the starter would perform in 2012. The consensus on Kuroda in pinstripes was a decent number two or great number three, but not many expected him to approach CC Sabathia's level of production. By October, Kuroda came within .9 wins of the southpaw's 2012 fWAR, and made 5 more games than the ace.
Despite how well Kuroda pitched in 2011, he should not have pitched as well in 2012. As I mentioned, his ERA/FIP differential was fairly large (3.07 ERA v. 3.78 FIP), Dodger's Stadium and most of the NL West stadiums were very pitcher friendly, and he was 37 years old. Likewise, his left on base percentage was 80% in 2011, more than 10% higher than his previous three seasons. So how did Kuroda pitch so effectively in 2012?
If you look at the PITCHf/x data, the right-hander had much better movement on his pitches last season. His four-seam fastball had an additional inch of movement horizontally, and two inches of additional rising action. The sinker also had around an inch more vertical movement, with half an inch more movement into right-handed hitters. His slider and splitter showed similar improvements in movement, while velocity remained the same across-the-board. It's an odd development for a 37 year, but the Yankees did some tinkering with him all season.
In the first month and a half of 2012, Kuroda pitched 48.0 innings with a 4.50 ERA, and hitters were batting .277/.338/.473 off of him. After a start where he gave up 7 runs to the Blue Jays on May 16th, it appears that the Yankees moved his release point slightly closer to the center of the mound, and after that he pitched to a 2.99 ERA in the rest of his 25 starts. In the image below, the release points from the beginning of the season to May 16th is in black, while the release points from May 21st through October remain in color.
There were some clear changes to Kuroda's pitches that likely helped him have a better season than 2011. Going forward, Kuroda should be a similar pitcher to what he showed with the Yankees last year, but that's doesn't necessarily mean a repeat of his previous year. Again in 2012, his ERA/FIP differential was significant, (3.32 ERA v. 3.86 FIP) and his left on base percentage was 5% higher than his career 72.7%. This isn't to say that Kuroda will necessarily regress from these factors in 2013, but a high FIP and LOB% can essentially be signs of luck.
The other side of regression is age related. Kuroda is 38 years old in 2013, he's thrown over 600 innings over the last three years, and history hasn't been kind to aging pitchers with high innings counts. The starter still lives off a hard sinker, and if the velocity begins to regress, or movement starts to slip, things can fall apart quickly.
Overall, his success in 2012 was likely not a fluke, changes made to his approach gave him better movement, but age regression is something that catches up to everyone. At 38 years old, Kuroda has a good chance of maintaining his stuff, but when you add in his LOB% and possible fielding luck, chances are his ERA will come closer to his 2012 3.86 FIP than his 3.32 ERA. Some slight regression is in order for Kuroda in 2013.