Still different players and different trainers. Yes, Messi knows Tata, but a lot of things are changing tactics wise. Tatas has a very different philosophy than Sabella, and that means a lot of changes. That not something you learn just playing games.
Again, he is not injured just because you say so. He's right foot is swollen and that's it. AFA says hes not injured. International press says he is not injured.
Making a sandwich increases the risk of a muscle injuries. And I'm not even kidding. There are cases of professional athletes getting injured playing with their kids at home, standing up after a press conference, etc.
The point is that one flight makes very little difference in the chances of injury.
And how do you know that Messi doesn't want to be in the states spending time with his teammates?