Comparing his pitch data from this season to his lights out 2008 season, it seems that three major differences are apparent thus far:
1. He is throwing more sliders (57% vs 51% in 2008), but the amount of sliders going for strikes and the amount hitters are swinging at are nearly identical. The notable difference is that less batters are swinging and missing at the slider for about an 8% difference in WHIFF% (huge difference). That difference in WHIFF% has translated to sliders put in play, which is about 8% greater between the two seasons.
2. Hitters are swinging at his fastball less often, a difference of about 13%, despite the fact that he has thrown sliders for strikes 58% of the time, compared to 55% in 2008.
3. His changeup, which he used 5.3% of the time in 2008, has yet to be used at all this season. In 2008, he was able to get batters to swing and miss at that pitch 15% of the time.
So, what it seems to me is that hitters are sitting on his slider, knowing that he is going to use it a lot and figuring out that it is not as good of a pitch as it once was. It seems that he has either loss confidence in his change up completely, as he only threw 5 of them last season and that the change may have been an important part in keeping batters just enough off guard to allow for his fastball and slider to be effective.