The Blues attacking midfielder was linked with a move away over the summer but the Serbian defender feels that he is up there with the Premier League's elite.
The Spanish attacker was outstanding in 2012-13 season, scoring 19 goals and adding 35 assists in all competitions as the Blues won the Europa League and finished third in the Premier League.
Mata was linked with a move away in the close-season and has started just one of Chelsea's three league fixtures this season under new manager Jose Mourinho, with several players signed in his position.
"He is a player who was one of the best in the last two years," Ivanovic told the press. "He came back late from the summer but he's had the time to work now. He's a guy who is really important for us and for the team.
"It's important for him to train hard, keep going and use every chance. I always said that, for me, Mata is one of the best-quality players in the league. He has a brilliant touch and he's a brilliant guy."
Ivanovic is pleased with Chelsea's transfer dealings this summer, with Willian, Andre Schurrle and Marco van Ginkel all recruited while Kevin De Bruyne returned from a season-long loan at Werder Bremen.
"Kevin has refreshed our front line," the defender enthused of the Belgian. "He has some different qualities to the other guys, some important things, and I think he's one of the guys who is most important for us in the attacking [third].
"He is someone who can change a game, decide a game."