Is Clayton Kershaw an Ace?

Dodgers left-hander Clayton Kershaw is a special talent. The Texas prep product, taken with the seventh pick in the 2006 draft, used his searing fastball and knee-buckling breaking stuff to whiff over 11 batters per nine innings in the minors. He reached L.A. at age 20, and has since K’d more batters per nine frames (9.4) than any starter not named Rich Harden, Tim Lincecum or Erik Bedard.

It seems like Kershaw’s already an ace — after posting a 4.26 ERA during his rookie season in 2008, he pared that mark down to 2.79 in 2009 and currently holds a 3.06 ERA in 2010. But has the prized young starter made the sort of marked progress suggested by his ERAs? I’m not so sure.

Kershaw’s contact and swinging strike rates have, without question, improved. Opponents made contact against him 79% in 2008, 77.1% in 2009 and have a 71.6% rate this season (80-81% MLB average). Kershaw’s swinging strike rate has shot up from 8.7% in ’08, 10% in ’09 and 12.1% in 2010 (8-9% MLB average). Consequently, his punch out rate has soared — 8.36 K/9 in ’08, 9.74 K/9 in ’09 and 10.19 this year.

But those extra swings and misses have come at a price. Kershaw’s walk rate has gone from 4.35 per nine in ’08, 4.79 BB/9 in ’09 and 5.35 BB/9 in 2010. Taking a look at his splits, it becomes apparent that Kershaw owns lefties, but his performance against right-handers lags behind:

His xFIP against lefties is 3.05 this season, and 2.07 for his career. Kershaw’s xFIP against right-handers is 4.74 this year and 4.60 for his career. Despite walking well over five righty batters per nine frames during his time in the majors, Kershaw hasn’t been hurt badly by opposite-handed hitters. Why? a minuscule home run per fly ball rate against righties — 1.8% this season, and 5.2% during his career.

Pitch F/X data from Trip Somers’ texasleaguers site shows that Kershaw’s arsenal blows away same-handed batters, but he’s mortal against righties:

He still gets a healthy numbers of whiffs with the fastball against righties, but obviously nowhere near the stratospheric rate against lefties. Kershaw’s strike percentage dips a bit as well, but not tremendously so. The big change is that he swaps some sliders for curveballs against righties. That low-70’s deuce, once dubbed “Public Enemy Number One,” is rarely thrown for a strike and isn’t getting many swings and misses. Kershaw’s changeup has never been a big component of his repertoire, but he’s throwing it to righties even less this season — 2.1%, compared to 5.1% last season.

As a rookie, Kershaw’s xFIP (3.96) outpaced his ERA (4.26). Since then, it’s been the opposite. He had a 3.90 xFIP in 2009 (2.79 ERA) and has a 4.26 xFIP in 2010 (3.06 ERA). He does seem to pitch well from the stretch (career 76.8 LOB%, compared to the 70-72% MLB average), and better pitchers (particularly high-K pitchers) will tend to post higher strand rates.

However, as mentioned earlier in the look at Kershaw’s splits, the main reason for the ERA/xFIP dichotomy is an extremely low HR/FB rate — Clayton had an 11.6 HR/FB% in ’08 (right around the MLB average), but that mark fell to 4.1% last year and is 3.9% in 2010. Color me skeptical that he’ll continue to give up homers on fly balls at a clip that’s less than 40% of the big league average. It’s likely that opposing batters will find the bleachers with greater frequency in the months to come.

None of this is to disparage Clayton Kershaw — he’s still younger than some of the guys who will get drafted tomorrow evening, yet he has already established himself as a quality major league starting pitcher. Given Kershaw’s skills, it’s entirely possible that he makes improvements in his game and tames his wildness against right-handers. But, at the present moment, he’s still an unrefined talent with some blemishes to his approach. He’s very good — I just don’t think he’s great yet. Short of gains being made with his control, Kershaw’s ERA may be closer to four than three from here on out.