The 30-year-old left Guardiola’s side on loan during the summer, joining Everton and having an instant impact for the Merseyside club.
Grealish has scored one goal and provided four assists in his seven Premier League games so far this season but is ineligible to feature against his parent club this weekend.
Speaking to reporters ahead of the game, Guardiola hailed Grealish for grasping the opportunity at Everton with both hands.
“Obviously he is there. The impact has been massive from day one. He got minutes and minutes and this is what he wanted,” he said.
“The decision we had with him, because he’s a lovely guy, but he’s back to real business and he’s playing every game.”
When asked about Grealish’s ineligibility for the game, the Spaniard added: “They’re the rules. When I will be a CEO or president of a big institution in England maybe I will change it. If he played it would be nice but I don’t have any opinion.”