No one has seen the change in his development more than international team-mate and close friend Branislav Ivanovic.
"He was my best mate the last time he was here, he is like my brother," Ivanovic said. "Last time he was young, now he's a really proper player. He's come back after three years from a great period in Portugal, the people loved him there. He has different confidence and attitude.
"He is the one who is leading the game, he asks for the ball and looks to go forward. Defensively he has improved a lot, too.
"He can be a very important player for us. He has signed a five-year deal so his future is here.
"Step by step, he will be one of the most important players and can still improve a lot."
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