It's likely he would have been in the bullpen to start the year until Giolito went down.
Anyway, I just think it's likely a mistake of being too clever by half to fiddle too much this way. Yeah, he's been injured for parts of each of the last few seasons, and that stinks, but since it's pretty much unknowable what's going to happen to a guys arm...if he can start and you need him to start and he can be effective, you start him.
The league is littered with guys who are injured for a few years straight, and then break out for 150 innings plus. 120 innings, even 150 innings as a starter, are still on the table, FWIW, for Whitlock, if he only misses 3 starts here.
I guess what I'm saying is it's easy to fall for induction and assume that this guy has been injured in the past, he's therefore going to be injured in the future, over and over. That may be true, but even the guys who are chronically injured have the occasional season where they throw 28-30 starts. If we missed out on that because we wanted to avoid uncertainty and tell him, no way can you start, you're too injury-prone, I think that would be a mistake. A pitcher can still get hurt throwing max effort out of the bullpen, after all.
Anyways, if we even get 20 or 25 starts from Whitlock, the way he's throwing, that'll be more than worth it. He's been that good to start the year.