The Spain international dropped down the pecking order at the West London club following Mourinho's return last year.
Mata won the Player of the Season award at Chelsea during his first two years, but was eventually sold to Manchester United by Mourinho in January 2014.
"For better or for worse, we had no relationship, there was no dialogue," Mata told El Pais. "I was not happy, but always gave the most and respected my colleagues. You never know what will happen in life."
Mata now works under Louis van Gaal at Old Trafford, but continues to fight for a regular first-team place.