When you are talking about potential, it is always subjective. Some say that if you only have two effective pitches, you should be a reliever. But that doesn't seem to hold true in all cases.
My point is that an effective starter is so much more valuable than a effective reliever (if for no other reason than he throws more innings, someone that has good stuff should have to prove that he can not be an effective starter. He was one in the minors until he was derailed by substantial health problems. I would like to see if he can be one again.