I don't think he's unrecogniseable from the player at Chelsea. It's a bizarre revisionism, implying Mata was an unstoppable force at Chelsea. He had plenty of games where he was ineffective and subbed off, just as he has done for us. In Premier League games, Mata has matched his output at Chelsea for United, in significantly fewer games. United definitely overpaid for him, but he's playing just as well.
And many seem to be of the belief he played as a central #10 every game there too, and that really wasn't the case. He played on both flanks plenty.