Just because Mata didn't fit into Mourinho's system some ppl have now convinced themselves that he can only play No 10 for us. Where would Silva or Nasri play for us then? Would we also question what would happen to them if another manager joined? Mata's best position is No 10 but that doesn't mean he is useless in other roles. You just need a good manager who knows how to use your attacking players and of course you need a midfield who is a bit better than our current one. Just because we don't have that midfield and that manager yet, doesn't mean that we should call a player kind of useless in other roles when in fact he is a very versatile player.
At Valencia (where he actually played his best football, even better than at Chelsea) for the first 2-3 years he mainly played on the left for them while Silva occupied No 10. And even then at times he played as striker/second striker, but also on the wing/Left midfield in a 442 at times. Only after Silva moved to City in 2010 he moved to No 10 for a year, before he himself then moved to Chelsea, where he again played in different roles very successfully by 3 managers with different style of play (AvB, RdM, Benitez).
He now might or might not be one of the first names on the next permanent manager's team sheet, but please Mata didn't start playing football in 2013 when Mourinho joined Chelsea and if you haven't followed him before that and at Valencia then at least don't make a judgement on his suitability to play different roles based on Mourinho's decision to get rid off him or based on Moyes' use of him.