Guus Hiddink believes Chelsea were right to hand Branislav Ivanovic a new contract.
Ivanovic, who joined from Lokomotiv Moscow for £9.8m in 2008, has agreed a one-year extension to play on next season.
The 31-year-old has come under heavy criticism for his poor performances this term.
But Chelsea's interim manager said: "He's important, he is one of the main characters in this team. He has developed himself as one of leading people.
"I knew him from my spell in (charge of) Russia where he played as a central defender in Moscow.
"He went to Chelsea after that and has become one of the key members of the first team and the squad.
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"He's back (his level of performance) to where he once was. He can play easily in central defence, both positions."
Meanwhile, Hiddink can't guarantee whether Papy Djilodoji will ever play for the club again following his loan move to Werder Bremen.
Djilododji saw just one minute of action in the first team after joining from Nantes for £2.7m in the summer.
He added: "I don't know that (if he will play for Chelsea again). I think he is still under contract with Chelsea and for those (kind of) players it's important that they get minutes and play.
"He's gone to a league that I like and respect and very much. It's an intensive league and it's a good club. It's nice to see he will start to play now and we will follow him closely of course."
