The City winger was back in the team for FA Cup fifth round tie for the first time since coming off early in our Champions League victory over FC Copenhagen.
Guardiola said after the game that he feared he had suffered a recurrence of a groin injury and he will miss our clash with Manchester United on Sunday at 15:30 (UK).
But the boss doesn’t expect him to be back before the international break later this month.
“He will not be fit for this weekend. When he will come back I don’t know,” Guardiola said at his pre-match news conference.
Asked about being ready for England, he added: “No. I need to speak with [England boss] Gareth [Southgate], I think he will not be fit.
“He has to recover well. He has had a setback, he has to recover well so we can use him as much as possible.”
Grealish looked dejected when he was replaced and was consoled by his team-mates on the bench after suffering the blow.
But Guardiola says he is confident that he has the character to cope with another setback.
“Yes, he doesn’t have an alternative,” he said. “Setbacks in life are always there, it’s how you overcome them.”
The Manchester derby starts another busy month for City with games against Liverpool and Arsenal to come plus a Champions League second leg with FC Copenhagen and FA Cup quarter-final tie with Newcastle United.
While it a tiring period, Guardiola says he can have no complaints about having five days to prepare for the visit of Erik ten Hag’s side.
And he said his players showed their determination with their performance in the 6-2 win over the Hatters after a quick turnaround from the testing 1-0 win at Bournemouth three days earlier.
“Tuesday to Sunday is enough - it’s a dream,” he said. “We recovered well.
“The problem is Saturday to Tuesday with trips, there is less recovery time. The players were extraordinary against Luton.
“Two days off, rest, don’t see each other then two days to prepare for the game against United.”
