Mata left Chelsea for Manchester United in January having seemingly been frozen out by Mourinho, who returned to Stamford Bridge at the start of the 2013-14 Premier League season.
Having been named Chelsea's player of the year for two consecutive seasons, Mata played just 13 of the London club's first 22 games in the Premier League last season and Mourinho indicated on numerous occasions that the Spaniard did not work hard enough.
"For better or for worse, we had no relationship, there was no dialogue," Mata told El Pais when asked about Mourinho.
"I was not happy, but always gave the most and respected my colleagues. You never know what will happen in life."
Mata added about new United coach Louis van Gaal: "He has so much experience and I can learn a lot from him. He's tough when he has to be.
"He says things in a firm way, intensely, but then he's somebody who is very close with the players. He really likes to talk with us, to ask us what we think about the exercises that we do in training."
"Maybe I feel my values more in England than in Spain," Mata said.
"Now I have not [been] mentioned but am not looking for excuses or justifications."
