Mata proved last season that he is one of the best players in the Premiership. Tons of assists and loads of goals too, really important ones as well. Why Mourinho won't play him is beyond me, although Oscar has apparently been filling his boots with considerable aplomb.
Think that's it - Oscar has displaced him. Read the other day that Mourinho wants Oscar in the number 10 role, with two other CMs playing more defensively. Mata hasn't adjusted to that different role so hasn't been used.