The Argentina international has still yet to officially put pen to paper on fresh terms at Camp Nou, despite the club announcing nearly two months ago that an agreement is in place.
Barca are reportedly confident of finalising a deal in the coming weeks but, having already seen Neymar depart for Paris Saint-Germain this summer, Guardiola has warned that it is possible Messi - who will be available for an eye-watering £275m - could be next to leave.
Asked about the chances of the buyout clause being invoked, he said: "I don't know - someone might if they have the money and want to spend it. It might happen, who knows?"
Man City are said to be willing to hand over the large sum of money to land Messi should he give an indication that he wishes to reunite with Guardiola at the Etihad Stadium.