I am not part of United Gaf, nor am I saying that Rooney is indeed playing great atm, but comparing between the two players is kind of silly. Even if it had to be done, Rooney is much more useful player to the team than Mata is. Rooney can play so many position, know the team inside out, scores goals and contribute to the the team much more than Mata does.
As far as contribution goes Matas contribution as a no. 10 is on par if not better than Rooney. But the style of play when Mata is playing soo much more fluidic and fast. Rooney seems to have games where he just lumps it to the wings and slows play down a lot when he plays in no.10.