Messi has ended a 21-year association with Barcelona, who have been unable to put together a suitable financial package to retain their all-time top scorer.
The 34-year-old is widely expected to join Paris Saint Germain in France but a move to reunite with Pep Guardiola at Manchester City should also not be ruled out.
Guardiola and Messi spent four years together at the Camp Nou, winning three La Liga titles and two UEFA Champions Leagues among numerous other honours.
Manchester City have recently spent £100m on Aston Villa’s Jack Grealish and Guardiola says such a heavy outlay will make it impossible to bring in the six-time Ballon d’or winner.
"We spent £40m on Jack Grealish. We paid £100m and brought in £60m,” Guardiola explained on the eve of Saturday’s Community Shield clash with Leicester City.
"He will wear the No.10 because we were convinced with Grealish and we were convinced Leo would continue at Barcelona, so right now he is not in our thoughts."
The former Bayern Munich manager added: "As a fan, I have incredible gratitude for the most extraordinary player I have seen in my life, for the titles he helped to [win for] Barcelona, to help me personally to go to Munich and then to England.
"More than that, the incredible amount of feelings and actions in front of the TV and watching what he can do with the ball, with his mates in one football game.
"Day by day, game by game, every three days he made something unique. All I can say is thank you so much for bringing Barcelona to another level, to allow Barcelona to dominate the world for a decade, and I wish him the best for the last few years of his career.
"He will not be forgotten. Hopefully one day we can say the best ever because he deserves it.
