The French left-back, who made a big impression in the early weeks of the season after joining City from Monaco, has said he hopes to return to action next month.
But Guardiola wants to err on the side of caution and feels he may have to hold the 23-year-old back for his own good.
"The most important thing after six months is to be careful," said Guardiola. "He believes, 'I am strong, I will come back soon, I will play like I did before the injury.' That is not happening.
"Even if he believes he is strong, he is not strong enough. After six months, he has to be careful."
The City boss said Mendy, who ruptured the anterior cruciate ligament in his right knee in September, was back in full training and could help out the team for short periods, following the approach taken with Ilkay Gundogan, who also suffered a serious knee injury, in December 2016.
The German was eased back into action at the start of this season but it was not until a year after the injury that he began to start games regularly again.
"We cannot forget Gundogan played in the past month a lot of games – I think all the games, 90 minutes, except the last one," said Guardiola.
"But before that, he played one game, rested for two, one game, rested for two, because after six months, the next step is muscular injuries. You have to try to avoid that.
"Maybe he (Mendy) can go to the World Cup and come back next season stronger."
City head to Stoke on Monday looking to claim the first of the four remaining wins they need to secure the Premier League title.
Kick-off is at 22:00 (SA time).