Mourinho has been delighted with Mata's form this season.
At today's Europa League media conference, Mourinho was asked about their history: “You are not right. I didn't sell. My job is not buy and sell. My job is to coach, my job is to work on the pitch and advise my boards about the transfer policy.
"I don't buy and sell and never did it. Secondly, he was the one that asked to leave. When a player asks to leave you always have to think twice. Also, my idea of football with the squad at Chelsea was one thing and the football I want to play at Man United and the profile of this squad is a completely different situation.
"My project at Chelsea is a 'good player'. My project at Man United is a 'very good player'."
Mourinho continued: “Big differences. At Man United I have no players to play transitional football as the main objective. We are not so physically strong like we were playing in a defensive block. Like we did when we won the title 2014-15.
"One thing our players for that football and another to try in a different way to play. The football we want to play here is different and is very adapted to his qualities. We need players for his qualities and we found a natural habitat for his qualities with us."