The Spanish attacker was outstanding for Chelsea in the 2012-13 season, scoring 19 goals and adding 35 assists in all competitions as they won the UEFA Europa League and finished third in the Premier League.
Mata was linked with a switch to Manchester United in the close-season and has started just one of Chelsea's three league fixtures this season under new manager Jose Mourinho.
Ivanovic has tipped Mata to again shine for Chelsea though, hailing the 25-year-old as one of the best in England.
The Serbian defender said after his country's 3-0 World Cup qualifying win over Wales on Tuesday: "He (Mata) is a player who was one of the best in the last two years.
"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 additions in the transfer window, as Willian, Andre Schurrle and Marco van Ginkel all came in, while Belgian Kevin De Bruyne returned from a season-long loan at Werder Bremen.
De Bruyne has started in two of Chelsea's Premier League fixtures this season and has impressed Ivanovic.
"Kevin has refreshed our frontline," he added.
"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."
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