Terry’s influence was waning under Andre Villas-Boas, Robert di Matteo and Rafael Benitez, before the 34-year-old was revitalised by the return of his former mentor in June 2013.
Terry has been an integral part of Chelsea’s success this season, playing every minute of every game so far in a title-winning campaign for the Blues.
“I thought he was in trouble. I thought he was over,” said Mourinho ahead of Monday’s game at West Brom.
“He hadn’t played for the previous two years, two or three consecutive matches. He was not an option for the managers. He was injured. The managers preferred many other payers in front of him.
“I thought there was something wrong and that he couldn’t get back to his normal level. I think he knows that, without me, he probably wouldn’t be at Chelsea in this moment. But I also know that, probably without him, I wouldn’t be again Premier League champion.
“I always thought he was going to improve and I’d have a good player. But, in the end, he came to a fantastic level. He’s had a magnificent season.”
“Probably, with another manager he wouldn’t play. And without playing he wouldn’t have another contract.
“This is not about the club, but the managers. If the managers don’t want the player, and the player doesn’t play, the club has to open the door.
“If a player is not important, sooner or later he leaves. Under me, he has had already two new contracts.”
Mourinho comes up against the reigning Barclays Manager of the Year when he takes his side to The Hawthorns on Monday night, and Tony Pulis believes the Portuguese could land the accolade this time round.
“He has got to be very close,” said Pulis, who won the award as Crystal Palace boss last year.
“Relentless is a great word and I think you can put that next to his name.
“If it’s a football match they are good enough to play against the best teams, if it’s a battle they can do that.
“I’m not so sure he’ll be worried about that (not winning any monthly awards) when he picks up his medal. I think he will go down as one of the great managers.
Mourinho branded himself as the Special One when he first came to Chelsea in 2004, but Pulis insists he is still just a normal guy underneath it all.
“He has got an edge, the edge is he wants to win and knows how to win. He demands winning and I have never seen him work as a coach but I know Steve Holland (Chelsea assistant) and Steve thinks he is outstanding,” he said.
“You have got to want to win. He is very driven. At the beginning of this season I did a Chelsea home game and I stayed in the hotel they stayed in.
“I’d rung Steve and Mourinho came down for a drink. We sat there for a couple of hours and he’s just one of us. You get pigeon-holed as this special human being but you take that off him and he’s the same as us.”