Here's Keith Law's take. You know, a professional who gets paid to scout these players:
Rubby de la Rosa blew up as a prospect before blowing out entirely back in 2011, but he's back from Tommy John surgery and, by 2013, should be able to pick up where he left off when his elbow snapped. He has touched 100 mph as a starter and sits comfortably in the mid-to-upper 90s, offsetting it with an above-average changeup with good fading action in the mid-80s, and, before the surgery, could throw both pitches for strikes. His slider is hard but really short, 82-86, but it doesn't have a lot of tilt to it because he tends to get on top of the pitch, often coming out higher than he does on his fastball; his curveball, a pitch he seldom throws, is in the mid-70s, breaking down but without tight rotation.
Even an average slider would give him No. 1 or No. 2 starter potential, and I think he can get there if he can release it from a slightly lower spot, closer to where he releases the fastball. It's a huge arm in any role, and, as long as he's healthy, he should be able to start.