Depeneds if we can find someone to pay Mata's wages. People talk about the difficulty of selling Rooney, but Mata could be just as difficult for Mourinho to shift, given he is apparently on £140k a week. If reports are to be believed, that's the same as Ozil and only slightly less than Silva, both of whom are much better players.
http://www.totalsportek.com/money/manchester-united-player-salaries/
Mata's performances at United haven't justified his wages, and that's always a problem when it comes to selling someone. He's clearly a lovely bloke but noone's going to pay £140k a week for a nice guy, are they?
Look at the various options. The other five members of Europe's financial elite - City, Chelsea, Bayern, Barcelona and Madrid - don't need him. Inter and AC probably can't afford him, and he's too young (and smart) to go to the USA or China.
The only option I can think of is a return to Valencia, who seem to have more money than sense, a bit like a poor man's United. But whether Neville (or the new manager) would want him is doubful, given they're in desperate need of some stronger personalities.
The only option might be to let him run his contract down and leave for nothing in 2018.