The 26-year-old rejoined the Blues from Benfica in January and has been an almost ever-present for the west Londoners, and he picked up the man-of-the-match award in the Premier League leader's 2-1 win over Liverpool on Saturday.
And his manager Mourinho was effusive in his praise for the Serb, telling reporters: “He is the best player.
"He was signed in the most difficult market. Which is for the players who are coming in mid-season, so the last half of the season was for him to improve, not adaptation, as he was a Chelsea player before, but to improve.
"At this moment he is a giant, not for his size but for the way he plays. The man is a giant."
Mourinho also hailed the mentality of his side, likening them to the championship-winning teams he has coached around Europe and says they remind him of the Chelsea team he forged in his first spell at Stamford Bridge.
He added: "I was champion in different countries, with different clubs. And there are points which are always present.
"I had it in Inter, Porto and Madrid and I have it here. I had it here in my first spell and I have it now, and this is something that I always have, people who are willing to sacrifice for the team, people who are willing to push, push to help, so I am so pleased with that."