Oscar over Mata for the nr 10 role makes sense; Mata is mostly an end-product player, Oscar is more rounded(talented), but leaving Mata out of the side doesn't make sense to me. I think Mourinho is more concerned about the defensive side of the game, Mata is no Dirk Kuijt when it comes to carrying out his defensive duties on the wings, and Mourinho probably thinks it would leave Chelsea terribly exposed having both Hazard and Mata on opposite wings, hence his choice for the hard-working Schurrle or De Bruyne on one of the wings. Mourinho has never been the sort of manager that takes a lot risks; he's obsessed with balance, control, defensive discipline etc
