The problem is Zlatan and Ronaldo understand their role and they are listening to the manager. You don't see them doing whatever they want. Messi is way more problematic in terms of positioning. He just will do what he wants on the pitch and nobody can change that except by cutting him from the team in one way or another.
Messi wouldn't be a problem if he would respect his role. But he doesn't. Stay 80% of the time in the last third of the pitch and look for spaces there. But, no. If he gets bored because he doesn't touch the ball for a few mins, he'll fuck our shape up, and he'll drop near Busi to touch the ball. Like in high school games where the best player in the schoolyard takes the ball from anyone and everyone gives him the ball because he's the best.
He's already starting to give looks to Fati for not passing him the ball and taking shots. He still doesn't understand that the team should not revolve around him anymore. He doesn't settle to a role and works hard in that position, like Ronaldo and Ibrahimovic, who know they are forwards and work hard to stay fit and score goals even at their age because they are super-fit professional athletes. He moves everywhere, asks the ball from everybody, walks around however he feels like. And then everybody must give him the ball because he is Messi. He is, in many ways, a delusional child who doesn't do anything to adapt to his new condition and this is why his form is falling off a cliff.