We've already got a roster crunch going on, and we're winning ballgames with the current line-up. The only piece that really makes sense to give up on is Morales, and then that would mean that Vladdy switching between 3rd and DH with Dondaldson and Pearce (DH only, obviously).
I just don't see a situation where the best move is bringing him up, rather than developing him. Now, if Donaldson is out for a long time, AND another injury happens where he'd be playing 4/5 or more games, then sure, but I don't see the rush with our current roster. We don't have an obvious position of need at the moment (yes, there are positions that could be improved, but Diaz/Solarte/Gurriel/Travis is a solid enough left side of the infield until Donaldson is back that I don't see the need to rush him.)