There were reports the duo fell out, but Mata insists Mourinho gave him his blessing to join United - and even ordered him to be a big hit at one of his main rivals.
Mata said: "We spoke and he (Mourinho) said that he thought this was a good option for me, and I told him I thought this was a good option for me.
"We didn't speak for long. I wasn't playing, and to be honest, I am happy when I play football. "It's the thing I like to do the most. He wished me good luck and told me to be good, as did everyone at the club (Chelsea).
"I'm very grateful for my time at Chelsea and for all the trophies I have won. We had a fantastic relationship, I had great relationships with several of my teammates.
"They weren't just my teammates, but my friends. Fernando (Torres) for example. I was very saddened to tell him I was leaving him.
"It was tough to tell him I was going because he is a very important person to me, but we'll keep in touch because we aren't that far away from each other."