Grealish - who joined the Etihad for a club-record £100m last summer - has not enjoyed the most positive of seasons since arriving from boyhood club Aston Villa.
Speaking ahead of City's trip to Leeds United, Guardiola said: "He can [have an impact]. Nothing changes just because in that position Riyad has been with us for a long time and in important games he gives you something unique and special.
"And Phil Foden is so determined, his will and many things. Phil, always, his impact in the game is huge - it's just for that reason."
Raheem Sterling is another player struggling for minutes, and Guardiola stressed that neither he nor Grealish were "bad" players and offered other qualities that he requires in certain scenarios.
"Raheem - everybody knows how important he is for all of us, especially for me," he added. "But in that moment I decide for those players, but not because [the others] are bad. I always have the feeling with Phil or Riyad [that] the goal is there.
"[They] create chances, when [they're] in front of the goal they have the sense to score a goal. It's one of the reasons why.
"I was judged at Crystal Palace on making substitutions for that reason. They were playing and had the feeling that they can score a goal when we need it. Sometimes we need another type of game, quicker or more control - sometimes we need maybe Jack, maybe Raheem.
"Most of the time it's a tactical decision, not because I'm unsatisfied or they are not playing good. That is the reason why."
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