Mata playing right - it's not a natural position for him. Ozil would probably play that role better I'd say. Mata needs someone like Rafael to come steaming down the wing to give him an option otherwise he'll get cramped for space and lose possession. No coincidence that he supplied the assist to RVP centrally. Same situation as Kagawa, plays poorly on the left/right, more effective behind the number nine.
If I had a choice between Mata and Rooney as number ten, I know who I'd pick. I doubt the guy reportedly considering £300,000 wages would have been able to play that pass to RVP. Rooney has been brilliant this season but Mata, with his quality, age, experience is the future for the number ten role. But Moyes seems keen not to upset Rooney and the club even, they're bending over to let Rooney and Stretford gain as much money they want.