The Blues defender failed to play regularly under Rafael Benitez last campaign, but has been a key player for the Portuguese so far this season.
"Yes, I had to remind him of his importance to the club when I came in because his self-esteem was a bit affected in relation to last season," Sky Sports News quotes him as saying. "He was a little affected, feeling in a quiet way pain over the doubts about him.
"I'm not here to give him something he doesn't deserve, but I told him if he worked hard from day one, he would play more than last season. He's doing well, behaving well and working well."
Mourinho was in charge of Chelsea between 2004 and 2007.