We are missing to many players.
As I said in another thread, Rooney is not a midfielder, he just isn't. He does as best as he can, but he is at least a class or 2 behind what is required (I am talking about CM position).
Carrick is tired, but we have no one who can play instead of him.
Now imagine Blind coming on for Carrick (new pair of fresh legs, plus someone who does really well under pressure), Herrera from start, Fellaini as an option in the box later on, di Maria who can run through their line and taking them out of position.
Mata would have some one to play with, our full backs would have more space, Rooney could play more forward....
So more speed, more options to attack, fresher players.... It usually makes a huge difference.