I kind of agree with DM
We need a Yaya Toure type but they are few and far between.
To be fair you need Mata as well. Ultimately if you bought lets say 3x Yaya Toure's and stuck them in midfield, then a Mata will do a hell of a lot more for the team than Valencia/Welbeck would, he IS the type of player you want.
So there is nothing wrong with getting Mata, I'm mostly getting at that he has a set of circumstances that allows him to be at his absolute best, but most players look better than normal in such circumstances. I just don't think he's worth 37million, he's not in that bracket. Given the choice I'd be spending 37mil on a Yaya then adding to the team by getting Mata for 20mil or so.
Thing is though even with Yaya, it doesn't have to be someone his level to drastically improve your midfield, look at Flamini, or Arteta(he was great his first year, kind of gone downhill since then, still decent, just not close to that first season). There are loads of players in all types of position where some are the 89 minute performers, and others are the 5 minutes of something special and 85 mins of anonymity. Hazard cost less than Mata yet does so much more in a game, the Southampton squad is full of such players who were basically all cheap.
You don't have to be able to run the length of the pitch and score a goal like Yaya does now and then, to put in a 90min performance that gives the base for other players to make the difference, UTD desperately need those guys. Again Mata would be great to have when Utd get some of those players in, in a wide range of positions. But spending almost 40mil on a player who mostly relies on others to make space for him or win corners/freekicks for him to take, I just don't think he'll have a major effect on the team as it stands.