Until someone from the playing staff come to comment about this situation, we can only speculate what has happened and I think there is more to the story than we know. From Mata's point of view, I don't think he hasn't showed any fight to win a spot in the starting XI. He has but what one can do if he is just not quite the right fit in the manager's system? Mata has improved on his defending and has tried to adapt his game to fit into the system (some might think he hasn't just because he hasn't run around, tackle, attack at pace like Willian or Hazard) but what is the point of that if the minute he does something not right in the eyes of Mourinho, he's taken off? He's not exactly one that is going to go around tackle, press, run at pace etc the whole time. It's just that his style of play ultimately is not compatible to what Mourinho wants, even if he has tried to adapt. Not to mention, when you consider the number of times he has started but then substituted later on, the frustration is bound to increase and get to him. Being the World Cup year as well is not helping since he wants to make sure he plays enough so that he will be picked by Del Bosque.