He'd be a huge improvement going forward on Valencia.
Whilst I'm in no way shape or form saying Mata is anywhere near Messi as a player, could they link up similar to how Messi does with Alves? Or more, how he used to when he was mainly on the right. Messi (Mata), drifting infield playing as more of a #10, with Alves basically playing as a winger?
He'd also be huge freaking improvement defensively, with Tony having no clue where to position himself for half the time.
ALves would be perfect. Great attacking threat but also good defensively. And that for free. We would be mad not going for him.