Considered one of the untouchables in Mourinho's first spell as boss, 32-year-old former England captain Terry will now have to compete for his place like everyone else and earn an extension to a contract which is entering its final season.
"He's not safe, and he knows he's not safe," Mourinho said.
"The only thing he knows is safe is our friendship, our relationship, but professionally, nobody's safe.
"I need to be honest with everybody and I think he appreciates that. We spoke about it and he understands perfectly.
"We used to say in Portugal: friends are friends, but business is 'apart'. This is kind of a business. In football, you have to prove yourself every day and every week.
"I trust him completely. I know he's a very good player, but he has to prove himself like everybody else."
Frank Lampard's contract saga rumbled on for much of last season before being resolved by a one-year extension.
Mourinho is not concerned about a similar situation troubling Terry.
The Blues boss said: "A player in the last season of a contract has to earn the next contract. That's very, very normal.
"I've spoken to John about that. John has to work hard, to give everything for the team and give the best of himself. And at the end of the day - I don't say the end of the season - him and the club will arrive at a conclusion."
Injury problems last term contributed to Terry dropping down the centre-half pecking order under Rafael Benitez, a decision Mourinho understood.
Yet the Portuguese believes the man who lifted the trophies as he became the most successful manager in the club's history between June 2004 and September 2007 has more to offer.
Mourinho added: "I feel the motivation is there, the pleasure of training and playing is there.
"His class is there. His position is there. From a physical point of view, he needs to work hard and not have injuries because injuries break the evolution of the player. For me, that's the key point for John."
A core group formed the spine of success in Mourinho's first spell in west London, but there are no untouchables now.
"That word comes at the point where the work they do makes them untouchable - and that takes time," Mourinho said.
If fit, Terry can be expected to partner David Luiz in defence.
Chelsea dismissed suggestions Luiz would be used as a make-weight in the bid to capture Manchester United striker Wayne Rooney and Mourinho believes the Brazil defender to be key in future.
"He has an important quality for the football we want to play," Mourinho said.
"He builds from the back and is comfortable with the ball. He's very important and different from the other central defenders we have from this aspect.
"I think he can improve, as everybody can. From a defensive point of view, he can be much more effective, much more solid. But, for me, the potential is incredible. He has to play always."
Mourinho is happy with his squad and dismissed suggestions of a bid for Roma's Daniele de Rossi, expressing satisfaction with his midfield options: Lampard, Michael Essien, Ramires, John Obi Mikel and Marco van Ginkel.
There is plenty of young talent in the squad and Mourinho is happy with the balance.
"They can be a very good group," he said.
"The older guys here, they are a good support, good examples, work very hard, very respectful.
"I think for these kids to be with Lampard and company is also big help, a big support for me."
If there was one criticism of Mourinho's Chelsea first time around it was that substance was favoured over style.
With the likes of Juan Mata and Eden Hazard at his disposal now, that criticism may be a thing of the past.
"It's difficult to be champions playing bad football," Mourinho said.
"It is difficult to score lots of goals with bad football. It is difficult to be very strong defensively with bad football.
"Good football is always this. We want to play good football, we want to defend well, we want to score goals, we want to play fast and intense.
"We have some players in the attacking areas - especially the three boys behind the striker - we have five, six players with great talent.
"We have to play adapted to their qualities. If that quality is to have the ball, to have control of the game, to have control of the possession. It is what we want to do."