Grealish has not featured since our win over Bournemouth earlier this season but was back on the bench for Saturday's 1-1 draw with Aston Villa.
With matches coming thick and fast for City, Guardiola is happy that the England international is available for selection again.
Asked how crucial Grealish could be, he said: "Very important."
The manager was also asked whether he considered using Grealish at his old ground on Saturday.
"We thought about it at one-zero but he trained just two days," he said.
"If we had scored two, for sure we would have had him in. Jack started the season really well and we need players so it’s good to have him back."
Watch the video above to hear everything Guardiola said in the second section of Saturday's press conference...