I do like Mata, but don't think he's worth 37mil. You can see just from that video, a few nice goals and assists, but so so many of his assists come from corners/free kicks, and not because of epic delivery but because Chelsea's movement at set pieces is probably best in the league and heading is very very strong.
Most of those corners, nothing special about the delivery, players move where it's coming down. Quite a few of the other assists in that video were very basic passes, say a basic midfield pass then Hazard does something special, beats three players and scores, a free assist.
If anyone else took corners/free kicks, they'd have gotten most of those assists instead.
I remember the Arsenal game last year in which frankly Mata was woeful all game long, every time he tried to get up the field he was under hitting passes, losing the ball, poor defensively yet rated as man of the match because 2 free kicks that I believe others won turned into assists.
I think he's quite heavily over rated, that doesn't mean he isn't a very good player, I think he is, he's just not almost £40mil's worth.
He's one of those players that needs pure quality around him to look that good, rather than the guy that inspires that quality in others. IE he's not a Ronaldo/Hazard. Problem being Utd really need the driving force of a player, not the guy who only shines when everyone else around him helps make him look good.
I'd say he's worth 20-25mil, will be interesting to see what he does at Utd, both if he's not the main free kick taker and with the current Utd team/Moyes tactics and style. There will be games in which Utd should have won but not having enough quality on the pitch hurt them, where I think Mata can be that calm head and nice little finish, but other games in which they've been out possessed, out hustled where Mata will just not be able to get into the games.