Like I said before he's still very young, I mean look at Matic's development. Holding onto balls under pressure is the only area which he has to develop crucially imo. When you play in the center, there are bodies everywhere and if you're soft like Mata you lose the ball or pass out quickly because you're not confident enough to keep it or win a foul.He already can't dribble, he has little chance of developing that part of his game, but if he doesn't learn how to use his back to shield the ball in tight spaces, sooner or later Mourinho will stop playing him against teams that press us.