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 but Ivanovic told the press: "He is a player who was one of the best in the last two years.
|8/1||Juan Mata is 8/1 with Coral to score the first goal in Everton v Chelsea
"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 deadlings 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 frontline," 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."