Grealish was a key player in City's treble-winning campaign last season, but his performances have been underwhelming with three goals in 26 games in all competitions this season.
The England international has not featured for City since picking up a groin injury in a 3-1 Champions League victory at FC Copenhagen earlier this month and was left on the bench in a 1-0 win over Bournemouth on Saturday.
"He is the same player, he has the same manager, and the way we play has not changed. It's just the way he has performed. That's the difference," Guardiola told reporters on Monday ahead of City's FA Cup fifth-round clash against Luton Town.
"I said from day one, we need him. He has a special quality for our team. But it depends on him. Hopefully, he can do a good last three months."
Guardiola added that he could not afford to give injured players time to ease themselves back into the team, saying: "They have to find the rhythm to play for 20 minutes or 90.
"At a high level, the team don't wait to be fit. You cannot give someone three or four games to be fit. What about the 10 who don't play? They deserve not to play?
"You have to see the training sessions and all the small details. The players don't have to convince me. They have to convince themselves that they deserve to play."
City travel to Luton in the FA Cup later on Tuesday, before hosting rivals Manchester United in a league clash on Sunday.
