Why would Chelsea sell? In January? To United? A player the fans love? The only way this will work is if a) Chelsea consider United to be so far behind the title-chase that they no longer consider them to pose any threat even with Mata, and b) Mata agitates for the move. It would be a major coup for United to land him, and crazier things have happened during the January transfer window, but I will believe it when I see him in a red shirt.