Guardiola says the defender cannot play three games a week for the season.
Gvardiola has featured in six of the last seven games for City and his manager said: "Josko cannot play every three days, we need Nathan. Otherwise, we cannot compete until the end.
"The most important thing is not training sessions, it is being fit.
"If you are injured you lose eight games. Muscular, not the injuries to Oscar (Bobb) or Rodri. No injuries is the only reason why you can stay until the end of the season.
"That's why the doctors, the medical department, the physios, the intensity you have to do, you have to be so careful because of all the travel and national teams and games every three days.
"I just pray for them to be fit, the rest will come. Talk a little bit, they get it and try to do it. The problem is how can they do it if they are out for one month and a half."