The Spain international was expected to be one of the first players through the exit door after Mourinho's appointment as new boss at Old Trafford.
A number of top clubs, including Everton, were monitoring developments but Mourinho - who sold Mata to United in January 2013 because he did not fit into his plans at Chelsea - says he is in no rush to offload him again, providing the player accepts he may not play every game.
GETTY
Juan Mata looks set to stay at Manchester United under Jose Mourinho
He said: "I sold him because he asked for that. Nobody in my previous club wanted to sell or push him. It was his decision. He was not the first option, he was not starting every game but I never left Juan Mata out of my selection of the 18 - but he was on the bench a few times.
"We had a proposal from a big club like Manchester United. We thought in the club that the proposal was very much acceptable, that it was a very good proposal for that period three years ago. But he wanted to leave and my philosophy is I don't want players who want to leave.
"I want players who want to stay. So we sold Juan but I think since the first day here everything is clear between us. There is space for him. He's a talented player.
"I don't promise places to anyone but I promise respect and I like him so if he wants to stay - and I think he wants to because up till this moment I have not had one little sign that he wants to leave - then, yes, he can be of use to the club."
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