The Citizens played out a 0-0 draw against Arsenal in the Premier League on Sunday.
While Grealish made a positive impact off the bench for City, Guardiola did not appear impressed.
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A video on social media showed Guardiola gesticulating and pointing out something to Grealish.
The attacker appeared to nod in agreement, with Guardiola eventually patting him on the head.
City are third in the table, three points behind leaders Liverpool and a point behind Arsenal.