The Diamondbacks starter tonight, Miley has struggled this season, to put it kindly. (Although, despite his struggles, he has still managed 8 quality starts out of 15.) His struggles, as was pointed out in the series preview, have occurred when facing hitters the second and third times through the order. This season, his slash line against the second time through the order is .371/.403/.613, compared to an excellent .168/.257/.248 the first time through the order. (His third time through the order is better than his second time, indicating that these figures are inflated by starts where he struggled in the middle innings and didn't get a chance to go through a third time.)
The comparison was drawn to Josh Collmenter, who had similar problems from the beginning of his career, which has led to a life in the bullpen. Collmenter, even in his excellent 2011 season, struggled the second time through (.305/.346/.479) and while the average against dropped the third time through, OBP and SLG stayed elevated, which would be expected of a pitcher tiring and losing some control. In his career, hitters are .304/.347/.533 against Collmenter the second time they face him.
By contrast, Miley's career figures are very different. Even including his poor figures this year, batters are only .290/.333/.455 against him the second time around. He's actually much better than his career numbers the first time through the order this season. Last season, he posted very similar numbers the first and second times through the order, with opponents batting 6 points lower the second time around. Also, in 2011 (small sample size, of course, with only 7 starts) Miley was significantly better the second time through the order (hitters were .273/.322/.400, compared to .321/.409/.544 the first time.)
In conclusion, Miley is not Josh Collmenter. What's wrong with him? I'm not sure. His BABIP against is higher than average (at .320 for the year, .335 excluding his first three starts, all of which were excellent), but not high enough to make such a big difference. He's giving up more line drives, but only slightly. I don't watch enough to analyze his approach, but I would theorize that either something in his pregame routine has changed, or his approach to hitters the second time around needs to change (because hitters are adjusting). That said, in 15 starts, Miley has given up only 14 runs in the first three innings, and seven of those were in one game (Wrigley with the wind blowing out). So while I hope he figures his issue out soon, at least we can be fairly certain of a good three innings out of Miley. We need six, but after some dismal performances of late, I'll take what I can get.