Mata's height isn't even the issue. Hazard is the same size and Willian isn't much taller, but they're much quicker, stronger and have better stamina which allows them to play the way Mourinho wants.
I feel sorry for Mata because he has tried hard as fack in some games to play the way Mourinho wants but he's just too small, weak and slow. The manager clearly isn't going to give him a free role so he's going to sell him.
I really think Mourinho wanted to sell him in the summer but the supporters would've went nuts so he had to give it a year before offloading him, then he can say I tried