The reigning AL Cy Young award winner has been, well, less than spectacular. 2-8 with a 3.9 ERA and as I write this he's given up 4 more runs over 6 innings. What's been the difference for Greinke? As dominant as he's been in the past, he's simply not dominating the competition like he did last year. So let's take a look at his performance this year compared to last:
2009: 2.16 ERA, 2.33 FIP, 9.5 K/9, 2.0 BB/9, 0.43 HR/9, .313 BABIP
2010: 3.94 ERA, 3.87 FIP, 7.9 K/9, 1.8 BB/9, 1.21 HR/9, .314 BABIP
So the difference appears to be pretty simple. He's striking out less batters than last year, but at a rate in line with his career rate, where he's been a mid-3 FIP pitcher. The real problem is the home run rate. He hasn't given up home runs at a rate like that since 2005, where he went 5-17 with a 5.8 ERA. His HR/FB% is up to 10.1%, which is right about the league average of 10.7% and is also the highest mark since 2005's 9.6%. Another main problem is his LOB%, which has fallen to 71.7% after last year's 79.3% and an average of right around 76% his last few years, which shows he's been a little unlucky.
The first reason I thought to check for his lull was simple. His slider was far and away his best pitch last year, and I thought maybe batters were learning to lay off it. However, it's worth about the same this year, 2.81 wins per 100 sliders as compared to the 2.9 mark last year. Pitch values, however, showed that his curveball has been absolutely terrible this year, moving from .5 wins/100 last year to a staggering -3.59. Digging into pitch f/x, his curveball is breaking about half as far as it did last year, not breaking out of the zone hard enough. The real problem this causes is shown in his opponents' contact numbers. He's getting way fewer swinging strikes (6.6% as compared to 9.9%) and opponents are absolutely teeing off on his pitches out of the zone (72.9% compared to his career mark of just 59%).
Bottom line? Greinke has been a little unlucky this year, but he appears to be having an off year. He's not fooling batters anymore with his curveball. I'd like to see individual contact numbers for his curveball, but I don't know where to find contact numbers per pitch. Greinke probably isn't as bad as he's pitching this year but he's not as good as last year either. If he gets his HR/9 back to about 8 or 9% and gets his curveball breaking like it used to, he can probably continue to put up an ERA in the low to mid 3s, but he's not going to be winning any more Cy Youngs I don't think, sadly.
edit: I found Texasleaguers, which has the data I was looking for. Greinke is getting balls in play on 23.5% of his curveballs, up from 17.5% last year, which in addition to the huge drop in break makes me think something might be off with his mechanics on the curve.