First thing is that the staff is different.Allegri brings his own staff.Maybe we can all recall what was his injury record at Milan.Pato leaves due to that injuries .Also Robinho was quite often injured.I think our training /medical staff is responsible for this.No one can claim that trainings are harder with Alegri than under Conte.No way.So it's staff's failure.
Well he's our only certain performer in midfield for us atm, and especially crucial against a team as good on the break as Manchester City. Of course you could argue about the exact dimensions and how you imterpret "a lot", but it's going to make the game significantly harder for sure. Who do we have that's capable of playing the DM position now? Probably Hernanes, which isn't too bad offensively except that he's two levels below defensively.