Bearing in mind that 'overrated' doesn't mean 'rubbish' -
Is Mata overrated? It depends who's rating is being taken into account.
Was his signing overplayed a bit? Yes, in my opinion. I could see from a mile off that this signing would do little to change our style/tempo of play. It was nice purely for the confirmation that Woodward is capable of completing a big deal abnd our dip hasn't put off top players. But this signing won't change our football without two centre midfielders.
Would we have been better off throwing the money at Matic for example (assuming he'd have come here)? Yes, and our football would be better for it.
However, it's still great to have him, and once we have a couple of midfielders who can be fully trusted on the ball, can turn on the ball and beat a man, can play a one-touch pass forward, and create space properly, the whole team (and especially Mata and Kagawa) will look so much more fluid and cohesive.
Throw someone like Ramsey in the side, and Mata would have looked a much better player, since his arrival.
Our football suffers because the key area for a passing game (the bridge between centre midfielder and number 10) is the one area our midfielders have no confidence and/or skill to play in.
Ramsey, Toure, Henderson, even Delph....
All pretty comfortable playing in that congested area; can all turn, beat a man, pass one-touch.
Nobody in centre midfield does that for us (only Anderson consistently tried), thus a whole area of the pitch is bypassed except for when the game is played at a snail's pace.
You see the same for England: If there's a man five yards in front of the ball carrier with a marker slightly near him (but with the space to potentially turn his man and be create a good opportunity), and a man sideways on the halfway line or a centre back behind - They'll never attempt the forward pass. Zero confidence to try the forward pass.
You only need to do this a few times a half, and your number 10 becomes redundant.
And it affects the whole team. Smalling looks horrendous at right back. But throw in a midfielder who doesn't just give it to him and leave him to it, and he wouldn't have to resort to hoofing it, due to being isolated.
Would Smalling look so poor with Scholes there? No.
There's nothing this team can do style wise - without a top midfielder or two.