How will this Willian transfer affect the development of young players like Van Ginkel and Oscar? If Mata is going to be playing then one of those might have to go out on loan. Might as well say goodbye to Moses as well. Can't see him playing at all.
Willian has nothing to do with MvG as he's been playing as a holding mid. Mourinho moved Mata from the middle just to accommodate Oscar, I think that says enough.