The Spanish international has played an instrumental part in Chelsea's performances this season and Mata has praised Di Matteo for his part in ensuring that the club's players maintain their form.
"His speeches in team meetings are very clever. And it's true that he knows how to go into each player. He talks with each of us about our rivals and how important it is to win trophies for this club," he told The Mirror.
"With me, he always gave me confidence. Of course, when you are playing well you don't need that.
"But when you are in a bad moment, you do need it, and Robbie knows when he has to say something and always gave that to me."
The 24-year-old has scored seven goals in 13 appearances for Chelsea this season.