The Diamondbacks, by any metric, have had a terrible starting rotation. It has improved of late, recently piling up quality starts. Still, though, no starting pitcher has an average game score as high as the league average (53). The highest is Collmenter's 52. What follows is a recap with projections for the future.
Wade Miley: Miley was the ace at the start of the year, forced into the position be default. He really hasn't handled it too well. He did post a quality start the last time out, but was getting hit pretty hard. He has fallen into negative value territory over his last few starts (now sitting at -0.2 bWAR and 0.1 fWAR). His average game score has been 49.4. The Diamondbacks are 4-4 in his starts. The good news is that his line drive percentage, BABIP, and ground ball ratios don't show signs of permanent regression. The issue with Miley appears to be command, which may be the result of trying too hard. He had similar issues last year but overcame them. Batters are fouling off a ton of pitches, leading to high pitch counts and making him work harder than would be ideal. Still, Miley should be solid going forward.
Brandon McCarthy: McCarthy started poorly, but has come around. Except that he really hasn't changed that much. He leads the team in ground ball rate, which shows that he is pitching the way he wants to pitch. But in his early starts, those ground balls were finding more holes. His BABIP remains .308, which is still high. Fangraphs (0.3 WAR) and BR (-0.1 WAR) are divided on how he has done, with Fangraphs rating him the most valuable pitcher on the team. His strikeouts have also increased. But the issue with McCarthy is the same as it has always been. Will he stay healthy? If he does, he will probably be the ace of the staff by default. If not, the D-backs will have to find someone to replace his value, and that won't be easy.
Bronson Arroyo: Arroyo has been a big disappointment. He might get a pass because of injury, and certainly his velocity has improved. He is developing into the innings-eater we thought we had. His last two starts have improved his WAR considerably; he's no longer bottom on the team, although still at negative value. His BABIP has been high while is line drive percentage has been league average, which shows a bit of bad luck. The fact is, he has the worst start on the team (game score of 4 against the Mets) and still has some work to do to recover from that statistically. FIP and ERA+ agree he has been terrible. One surprise is that his ground ball rate has been above league average. I think because of the injury, we need to see a couple more starts out of Arroyo before we can draw any conclusions, but I'm optimistic that 1) he's figuring it out, and 2) he won't be as horrible in Chase Field as people thought. While his ground ball rate may not be sustainable, four of his five highest ground ball rates have been in the last five years.
Josh Collmenter: Collmenter has been a pleasant surprise since moving to the rotation. He has done better the second and third times through the order than was expected, probably the result of developing a third pitch. According to Game Score, ERA+, and FIP, he is the best starting pitcher on the team thus far. fWAR has him second among starters with with 0.1, and bWAR has him first with 0.4. While it still remains to be seen whether or not Collmenter will continue his success in the rotation, I'm cautiously optimistic that he can reprise 2011.
Mike Bolsinger: Bolsinger is most likely out of the rotation after two subpar starts. His WAR is worst among the current rotation, and while his FIP is still better than Miley's, he's tied with Arroyo for worst ERA+ on the rotation. He has the tools, when hitting his spots, to be a decent 5th or even 4th starter. His line drive percentage is below league average while his BABIP is second only to Joe Thatcher's on the team (.367). So he's had some bad luck. He might have a future in the rotation, but it seems like that Chase Anderson or Randy Wolf will be given a shot now.
Randall Delgado: The team had a quick hook with Delgado, allowing him to make only two starts (with an insanely high BABIP) before pulling him. But he has done well in the bullpen, despite having some tough luck. He doesn't do well when pitching back-to-back days. This is problematic, as with multiple displaced starters in the bullpen, sometimes he'll have to do so. But he seems to be a victim of mismanagement more than anything else at this stage.
Trevor Cahill: Remember how bad Cahill was in the rotation? That's about how good he's been since moving to the bullpen. He's had one rough outing, but has looked really good otherwise. He might get another shot at starting if Anderson/Wolf don't work out, but I expect he'll stay in the bullpen for a while, given his relative success there.
At the moment, I'm confident in most of these pitchers to give the team a chance to win. Even last night, the LOB monster was equally a culprit with Bolsinger/Delgado, as the D-backs should probably have scored at least six off of Garza. This isn't a strong rotation by any means, but neither is it terrible. Barring injury, four out of five starters are going to put up a solid performance most of the time. If one out of Anderson/Wolf/Cahill/Bolsinger can come through, we as fans will need to look elsewhere to find a place to blame. Situational hitting seems like a good place to start.