Terry, who won two Premier League titles under Mourinho during the Portuguese manager's first spell in charge at Stamford Bridge, is about to enter the final season of his current Blues contract.
Now 32, the former England captain struggled to hold down a place under Rafael Benitez during an injury-hit 2012/13 campaign.
Speaking from Chelsea's pre-season tour in Kuala Lumpur in Malaysia, Mourinho was quick to encourage Terry and voice his respect for the centre-back, but could offer him no assurances over his future.
Mourinho said: "He's not safe, and he knows he's not safe.
"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."
Mourinho says he is unconcerned about the prospect of another contract saga similar to that experienced by Frank Lampard last season.
The Blues boss said: "A player in the last season of a contract has to earn the next contract. That's 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."
Mourinho has no concerns over Terry's defensive qualities but acknowledges he will need to avoid a repeat of his injury setbacks if he is to challenge for a centre-back position.
He 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."