Two things to keep in mind:
1) Throwing out the whole, should he play the 3 or not, for a minute, the bigger issue is that I don't think Doc is going to go with a whole second unit, nor do I think he should. Right now one of the Big Three (if not another starter as well) is on the court with the "second unit." This really makes the team more dangerous, as even when things break down, Paul/Ray can still get off a shot no problem.
So the bigger problem with Davis isn't so much if he can play the 3, it's that in order for him to break the rotation, he's going to have to knock someone else out of it.
2) That said, I think his role when he comes back is water boy and cheerleader until he proves otherwise. If he can knock Williams/Scal out of the rotation, great. But I wouldn't hand it to him. There's no need to mess with what we have when what we have is working.