You really think that we would even consider signing him? Only two clubs come on my mind. City if he wants really new challenge and PSG if he wants to enjoy in football without pressure. Easy trophies, easy league to build up his stats and life in Paris.
PSG isn't the place for no pressure, the owners and fans put constant pressure on everyone. And I have no idea about what United would or wouldn't do when it comes to someone like Messi but to be honest I hope that he goes to the Old Boys, if I'm not mistaken he said that he wanted to finish his career in Argentina and play for his boyhood club, if he does it while being a top player I will have as much respect for him as it is possible.