Terry has been the mainstay of the Blues' defence since Mourinho began his second spell at Stamford Bridge over the summer.
The former England captain turns 33 on Saturday, but has already appeared in the same number of this season's Barclays Premier League matches than he did in the whole of last term.
Mourinho revealed that his self-belief had been damaged after being marginalised by his predecessor Rafael Benitez.
"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," Mourinho said.
"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 refused to put a timeframe on when Terry, who will line-up in Saturday's Premier League clash with Stoke, should retire.
"At John's age players can reach a level of stability when they remain the same - 33, 34, 35, 36," he said.
"I look at (Inter Milan full-back) Javier Zanetti in 2008 and in 2012 and he's the same. Other guys are not the same from one season to the next for some reason.
"Players have to enjoy themselves and play with the same ambition. They have to enjoy the moment and not be worried about one more or one less pound in their contract."
Chelsea are hoping to close the gap on leaders Arsenal when they travel to Stoke, with Mourinho insisting the landscape in the Premier League has changed this season.
"I was always in fights in what you call here in England two-horse races," he said.
"They were Inter and Roma, Porto and Benfica, Real Madrid and Barcelona, Chelsea and Manchester United.
"Two-horse races have completely different profiles, not just because you're competing against a direct opponent, but also because the two horses never lose."When it comes to four, five or six teams competing like it is now, every weekend a team loses points.
"You can't just look at your direct opponent, you have to look at the others too.
"When it comes to this type of league profile, the best thing to do is don't look to the others because they are too many, just look to yourself. You just try to win your next match."