Watching him pitch I just don't see how he would be a dominant starter. Unless of course there is some major progression in his secondary pitches.
Just to recap:
Fastball: While he can reach 97-99 he typically pitches around 95. Nice tailing action with some downward motion. That is when he throws free and easy. But when he bumps it up you can see the extra torque and you can also see a loss of control. No biggie. But this is while relieving. When he starts either he will tire in the fifth doing this or more likely have to either A) throw less velocity or B) throw more pitches with higher stress and lower accuracy
Changeup: Sorry KLaw and others. I just don't see a change-up. Yeah, he calls it that. But it's a split finger grip and it's a split finger velocity, and split finger tumbling action. It can be devastating. The problem with the pitch is two-fold. If you've seen a lot of Contreras you would know. One is that the pitch doesn't hump and tends to drop early. Once he's been scouted it's excellent on two strikes but not very good before then, just too easy to detect unless the hitter is forced to defend the zone. The second is it's a pitch that is inconsistent on when it breaks. You will see a lot in the dirt. Oh, should have mentioned a third. Not so good on the body.
Curve: Didn't like it, seemed more of a show me pitch. Ok show me pitch. But the only guys that I saw fooled were because they were expecting or cheating on gas and got burned.
Health: He had problems with his shoulder I believe in the minors. And he is a high pitch count guy. With a 3/4 arm slot which really doesn't bode well for improving his off-speed. He isn't the biggest pitcher. And he is much more of a max effort guy than people give credit for. Yeah, when he is in the bullpen he can throw 95 easy. But he can't throw 98 easy in the pen. And he definitely can't throw 95 late into a game easy as a starter.
I really so no reason to move him to starter. It's just not true that "it can't hurt to try him as a starter". With his body type he might only have 3 or 4 dominant reliever seasons. Do you want to waste that with mediocre at best starter numbers? I know I don't.