The England winger had to come off during the team's 6-2 win over Luton Town in the FA Cup fifth round.
He was replaced by Jeremy Doku after 38 minutes, as his previous groin problem appeared to reoccur.
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I didn't speak to the doctor yet but I think he complained about the groin, so maybe quite similar," admitted Guardiola to club media.
He felt really, really good but unfortunately he was injured again at the beginning of the game."
On Grealish being upset, he added: Of course.
It has been a tough season for him so now he has to recover well and help us when he is able to come back."