I would also choose Neymar to leave the team if there is really a necessity for one of the trio at PSG to join another team. Because Neymar don't have a good past with Mbappe in the team as well.
These guys are professionals and need to learn how to work with others, and they aren't the first to have dressing room wrangles as history has shown..
Besides if Neymar is to leave PSG what guarantees the team that the next player to replace any of these guys won't invite misunderstandings... I guess you get where am going with this
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They had some disagreements before which they have solved hopefully. But there is no guarantee that this would happen once again between them. As far as I know Mbappe has called him as undisciplined before. This might also be a reason to choose Neymar in this.
If this affects their performance, management will jump in no doubt but seeing both are getting on the scorecard and the alike this is normal behavior for players that want to be the best!
There is still no news about Messi's decision on his future as well though. If he doesn't extend then maybe he will leave the team first. It is really hard to make a prediction about this thing now.
Messi is a professional that won't allow himself to start speculating about his future, but when the time comes he will let the world know... But I would like to see him follow Cr7 for a good fat paycheck and then retire from home (Argentina).