Following the 51-year-old's return to Stamford Bridge last summer, the Spaniard struggled to secure regular first-team football and made just 13 Premier League appearances before his switch to Manchester United at the end of January.
Mata was named as Chelsea's player of the year for two consecutive seasons before Mourinho's arrival and the midfielder says that he did not ask why he was frozen out.
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"When I played, I played. He would give me instructions.
"There are coaches who explain but others who don't think it's necessary. That's all."
Mata maintains that he still keeps in touch with his former team-mates at Chelsea but concedes that the build-up to his move to Old Trafford was difficult to cope with.
"Mentally, it was complicated. It was hard to leave Chelsea and I'll always be grateful but, at the same time, I was joining a great club,” said the midfielder.
"I spent virtually the whole time at home with my family, trying to stay calm, waiting, watching time pass, watching films and series, checking the phone. I went to the gym to make sure I didn't lose shape because I hardly trained [with Chelsea].
"Lots of team-mates are friends and they call. Not to gossip but to ask how you are, to be by your side. I'm still in touch with people at Chelsea; there are good people in that dressing room, friends forever.
"Everything depends on the club more than the player: the quantities, the paperwork."