I don't disagree he sucks. I don't think it has anything to do with him being a reliever though. You keep saying he was fine in spring training, but really he was awful in his starts in spring training and leads me to believe he just doesn't have it right now whether he is starting or relieving.
I do think his back injury is probably a significant factor, and he may get better, or may never be the same pitcher.
He started three games in ST. The last start was 3.2 innings, gave up 3 hits and one walk. Clearly an improvement. He made four more relief appearances and only had one bad outing. His last two relief appearances, he threw 9 pitches (all strikes) and gave up one hit. I'm not buying the he's still hurt stuff. Quite frankly, if you're still hurting months after surgery, you aren't ready to play. In fact, that's grounds for malpractice suit. What your suggesting is that 9 months after he knew he had a serious back injury, he's still experiencing the pain involved with that injury and one of the following three things is happening:
1. Detwiler is heads and shoulder the greatest at tolerating pain and not showing it.
2. A doctor knows about his condition and has not told him the implications of continuing to play. This is grounds for revoking a medical license and suing the everliving crap out of a doctor.
3. Detwiler knows the implications of continuing to play and is choosing to.
I find all of these incredibly doubtful. The only logical conclusion I can come to is that Detwiler is not injured.
He sucks. He knows he sucks. He doesn't want to be a reliever and he doesn't know how. Either give him a start, evaluate him from that, or DFA him. Treinen certainly hasn't shown that he's any better than what Detwiler can be as a starter.