First of all Chelsea didn't rip us off, they didn't sell us a player who is considered deadwood by any means. Everyone knows how Mourinho's teams play on the pitch. They defend very deep, leaving no breathing space in between the lines and they are very rigid which means that they can not afford players who can't do their share of defending because that could lead to defenders drifting out of position and the whole team losing its defensive balance. It's true that Mourinho didn't have a place for Mata in his plans but he didn't want to sell him to a rival club. If it wasn't for FFP Chelsea probably wouldn't have sold him to us.
The same thing is happening at United. We can not afford to have a #10 in the starting lineup, no matter the formation, right now. Even if Silva came here he would also look rubbish. Doesn't Silva lose the ball, doesn't he make mistakes? Yes, he does. Does he even bother to track back and defend? No, he doesn't. In both cases Pellegrini doesn't give a flying rat's arse because he can protect him with two massive CMs playing behind him and with an incredible CB pairing that doesn't allow anything or anyone to get past them. Only in CL matches against sides who can defend effectively and can keep the ball City face serious problems.
Mata is good when he receives the ball in the final third and has teammates around him making themselves available for a pass. When you can't commit enough players forward because you know that if you lose the ball your opponent can cut through your midfield like a knife cuts through butter, Mata becomes anonymous. Asking him to drop in deeper areas won't work either because if Mata could operate in those areas Jose would have kept him in the team, he would have bought someone like Vidal, instead of Fabregas, and he would be playing now the way he did at Inter with Sneijder.