Same dilemma in my roto league. I'm WAY behind in ERA and WHIP (and IP) thanks to the likes of Kaz, Nolasco, Wang, etc. at the beginning of the year. On one hand he's looked great in his rehab starts, but minor league players are in the minors for a reason, right?
I'm not 100% sure he's going to be any good, period. His advanced numbers have been worse year after year, so it seems like it was just a matter of time before he had a bad season. On the other hand, he's been a very good pitcher fantasy-wise aside from this year.
I think I'm going to start him. Even if he's not great, he should be able to K some people and I like his chances for a win. I will definitely be holding my breath, though, and if you're in a position where you don't need to start him, I certainly wouldn't blame you.