The Spain international's future at Stamford Bridge was reported to be in doubt earlier this summer but he has since held positive discussions with the returning Portuguese coach
Mata's future at the club was cast into doubt earlier this summer, with the Spain international demanding talks with the Portuguese coach at one stage.
The Spain international revealed that he did hold a meeting with Mourinho to clarify his status with the Blues, but remained "relaxed" regarding his role at the club.
"I was so relaxed about all the speculation about me leaving," he told reporters. "I spoke to the club and spoke to the manager who said I was very important to them. So there’s nothing to say about it.
"It’s always very good to hear you are important to the club. Everyone wants to feel they are important.
"So I’m very happy. I’m training with the new manager and we want to improve on last season."
Mata has been named Chelsea’s player of the season for both of the last two campaigns, and was one of Europe's standout performers last term, scoring 20 goals and registering 28 assists.
However, he acknowledges that competition at the club is growing as the likes of Oscar and Eden Hazard, who both arrived last season, are now ready to show their true worth.
"We just have to be focused," he continued. "There are 25 players in the squad and it is not down to us who the club will sign. We are happy with the squad we have at the moment.
"When you are playing more and more with your teammates, you get more of a connection. Players like Hazard and Oscar came last season and this summer there is [Kevin] De Bruyne and [Andre] Schurrle. All we need is the connection."