The 32-year-old defender was the man who lifted the trophies in Mourinho's first spell as boss from June 2004 to September 2007 and has been skipper since, but he will receive no favouritism from the Portuguese.
"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.
"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."