The Spaniard had been Chelsea's player of the year for two consecutive seasons when Mourinho rejoined the club in 2013 but found his opportunities limited under the Portuguese.
He moved to United in the summer of 2014 and might well have feared the worst for his prospects when Mourinho took over the Old Trafford hotseat this summer.
But Mata has started four of United's six Premier League games so far and was instrumental in their 4-1 victory over Leicester last weekend, scoring one goal and setting up another.
And Mourinho insisted that, just because Mata did not fit into his team at Chelsea, that does not mean the same will follow in Manchester.
He said: "I didn't sell (him)
My job is not to buy and sell
My job is to work on the pitch and advise my board about the transfer policy
He was one that asked to leave and, when a player asks to leave, you have always to think twice.
"My idea of football with the squad I had at Chelsea and the objectives we had at Chelsea is one thing, and the football I want to play at Man United, the profile of my squad and what I want to try to do in this club, is a completely different situation.
"In my project at Chelsea he was a good player, and in my project at Man United he's a very good player.
"The football we want to play here is different and I think it is very adapted to his qualities
I think he found a very good natural habitat to his football to play with us."
United appear to have regained their momentum following a three-game losing run with victories over Northampton in the EFL Cup and Leicester.
They will aim to take three points in another competition on Thursday when they face Ukrainian side Zorya Luhansk in the Europa League.
Their campaign got off to a disappointing start with defeat by Feyenoord two weeks ago, and midfielder Jesse Lingard feels victory at Old Trafford is essential.
He said: "That's the most important thing
We need to carry on like we did against Leicester, and if we can do that I'm sure we'll get the three points
It's mainly about momentum
The Leicester game gave us a lot of confidence as a team."
Mourinho is set to field a strong side but must weigh up whether to start his captain Wayne Rooney, who has been hampered by a back problem this week.
The United manager said he wanted to "protect" the striker, who was dropped to the bench against Leicester following heavy criticism of his performances.
Anthony Martial is fit again but Henrikh Mkhitaryan, Luke Shaw and Phil Jones will not be included.
Source : PA
Source: PA