It was one game, but this is Messi you're talking about. Best player in the world, arguably one of the best of all time, you know the one.
If he's made available to City then it's blank cheque time. You can cite his work rate from the PSG game, which admittedly was shocking, but that's down to Enrique mostly. He eschews tiki-taka and passing/movement in favor of a more direct back to front approach and generally keeps Messi lurking around the danger zones. Pep would play Messi entirely different.
Of course, it's highly unlikely he goes anywhere, but the financial constraints on Barcelona are starting to show and will only get worse if a new manager comes along with a list of demands for signings.