Which is precisely the point. You already have a player who is similar (and up until his move to United, was one of the best #10's in Europe), yet he has not been given the same amount of chances over the same period. Mata has done well for us, yes, but that doesn't mean that he won't suffer the same fate that Kagawa has should he move to United.
Regardless, I still find it really difficult to believe that JM would sell one of his most talented players to a rival. Regardless of what others say, United are still the champions, and directly strengthening them is never a wise move.