Mata, who was Chelsea's player of the year for the last two seasons, has struggled to break into the starting XI since Mourinho has taken over at Stamford Bridge.
The sporadic use of the Spaniard 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.
Mourinho hinted Mata, who wants to play in the 'number 10' role, had to be prepared to play in a wider role and says Oscar is the first-choice for that position behind the lead striker at the moment.
"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 analyse his performance. He came on against Basel when the team were winning 1-0 – not like Demba Ba or Mikel John Obi, 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 number 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 number 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."
Mata, who has been a key member of the Chelsea side that has won the UEFA Champions League and UEFA Europa League over the last two seasons, has featured in just three matches under Mourinho.
Along with Oscar and Eden Hazard, Mata was regularly part of an exciting attacking triumvirate behind a lone striker last season, and Mourinho says there is no reason that cannot be successful again.
"But only when (Mata) adapts to what we want," Mourinho said.
"I'm not ready to ask Oscar to track opposing full-backs. Brazil has more talented players in the number 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 number 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."
Mourinho also said the Blues slow start to the season, which includes back-to-back losses in the Premier League and the Champions League, is due to a change in the team's playing style he is trying to implement.
"I don't like the way Chelsea were playing the last couple of years; the club doesn't like it and we want to change," the Portuguese said.
"We want to play a different style. The past is history – even my past. I'm here like I've just arrived."
