The Portuguese boss says the 25-year-old playmaker must change his playing style or he will remain behind Oscar in the pecking order
The Spain international, who was the Stamford Bridge outfit's Player of the Year for the last two seasons, has struggled to break into the starting XI since Mourinho took over at Stamford Bridge.
The sporadic use of the 25-year-old has perplexed Blues fans but Mourinho has made no apologies for his decision and says Mata has work to do if he is to become a regular starter and usurp in-form Oscar.
"The way I'm reading the situation and the reasons why, in this moment, he's [Mata] not playing so much are things I can speak about with him but not publicly," Mourinho said on Friday.
"But he played against Everton from the start and you can analyze his performance. He came on against Basel when the team were winning 1-0 – not like Demba Ba or John Obi Mikel, who came on to rescue the game –with specific tasks to do.
"It's part of a process with him, too. It is one thing to play with Ramires and Oscar closing down opponents on each side, and Mata as a No.10, behind a striker with his clever assists, clever passes and fantastic actions because he has great talent.
"But it is another thing to adapt to the way we want to play. In this moment, Oscar is my No.10 and, if anyone tells me Oscar has not been Chelsea's best player this season, I'd have to disagree.
"I have to prove to the fans that I am good. Now [Mata] must do the same."
The triumvirate of Oscar, Eden Hazard and Mata enjoyed success last season, and Mourinho insists the trio can still be effective but only if the Spaniard alters his game to become more defense-conscious - though he is unlikely to start ahead of Oscar in the No.10 role.
"But only when [Mata] adapts to what we want," Mourinho continued. "I'm not ready to ask Oscar to track opposing fullbacks. Brazil has more talented players in the No.10 position than any other country in the world, and he plays there for the national team, so I want to build with Oscar as my No.10.
"I want the other two players, from the side, to adapt to that reality and learn how to do things they were not ready to do before. The players are open to some of the changes.
"If I'm an attacking player I'd prefer to play without a position, without certain responsibilities, and with others behind me to cover so I don't need to worry about [tracking back]. It's about changing mentality.
"It takes a bit of time. But it won't take five, six, seven years. I promise."