Mourinho sold the Spain international while at Stamford Bridge, despite Mata’s popularity at the club.
And the playmaker has become an important player at Old Trafford, having cost Manchester United £37million in 2014.
Under David Moyes and Louis van Gaal, Mata featured regularly, but there were fears that the arrival of Jose Mourinho last summer could spell difficulties for the 28-year-old.
Juan Mata has scored six Premier League goals for Manchester United this season
When I came here people thought that Mata was in trouble
However, even with the strong competition for a place in attacking midfield at United with the likes of Paul Pogba, Henrikh Mkhitaryan and Wayne Rooney, Mata has highlighted to Mourinho he deserves to play and stay by adapting well.
"When I came here people thought that Mata was in trouble," Mourinho said.
"I knew that he wouldn't be in trouble because one thing is the football I wanted my Chelsea to play and another thing is the football I want my Manchester United to play and Mata has adapted to the football we want to play.
"So I knew it since the beginning he would be an important player and I think he feels that confidence in me and he feels that confidence in the game we want to try to play.
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“So he's happy and he's happy on the pitch and you can see that in the way he plays."
Mata got on the scoresheet against Watford on Saturday in United’s 2-0 win.
And Mourinho could start him again on Thursday for their Europa League showdown with Saint Etienne (8.05pm).