He'd be great for about 18-24 months, then he'd lose the dressing room and it would all go tits up.
Also, I don't know if he has experience in building a squad from the foundations up? If we're looking at Mancini then we're aiming for the very very top, in which case who do we have capable of playing at that level?
Benteke, Guzan, and, erm...
You'd like to think if someone like mancini took the job it would then mean funds being made available to buy players capable of playing at that level.