The problem isn't so much about playing Mata out wide, though he's less effective there IMO, but more so on the defensive of things. Playing Mata on the flanks, especially on the RW, would mean he's always likely gonna come inside onto his left foot. Now, while that may look exciting with Azpi or Ivanovic marauding down the right side to create overlapping situations, the issue comes when the attack breaks down. Our RB may find themselves being overloaded in 2 v 1 scenarios, for example, and that could lead to us conceding some cheap goals, almost like when under RDM this season. And not to mention, there is more emphasis these days for full backs to go forward. Moreover, Mata doesn't have the pace and energy levels to be playing out wide as players playing there would usually have to bomb up and down the flanks all game long. And if Mata plays there, he's gonna be absolutely knackered by the end of every game. The best position for him really is where he's playing right now, as a CAM behind the ST. He connects our midfield and attack brilliantly and that role would also allow him more freedom to roam around on the pitch and create chances for others without necessarily worrying too much on the defensive aspects with two midfielders behind him. This is certainly better than sticking him out wide where he would have a bigger defensive duty and also the pressure to create chances from out wide.