If you look at the main stars of PSG, Messi-Neymar, and Mbappé we will see that all three of them are going to go quite deep in the World Cup with their teams.
Hakimi is also still competing in the quarter-finals, he is also an important player in PSG's defense at the moment, so don't forget him. Several other Brazilian and Portuguese players are also still in the World Cup, they are important players too for PSG apart from Messi, Neymar and Mbappe.
So, I think PSG might actually encounter a little bit of problems with the restart of the club competition.
I dont know if you are aware that Messi is injured or not. But he is determined to give his full potential to the national team, it doesn't matter if he gets injured or not. I dont know if he intends to take time off or not. But it doesn't seem unlikely that he will get injured.
Neymar already had an injury. He is playing for Brazil but I dont think he has made a full recovery. I expect Neymar to take some time off after the World Cup. Mbappe will probably stay healthy because he is not that old. But he can be fatigued.
The fatigue factor you think about some of these players may be true when the league returns. But in fact these players have a way to restore their condition and keep their fitness top-notch because they have enough practice. Indeed, they are not all robots without the fatigue factor, but PSG knows how to get them ready to start the league again and manage it later.
All the players who are still competing in the World Cup are still fine, but because there are still matches, the possibility of injury is always haunting them. Neymar has recovered, and I think he will be fine now.
Three of PSG's best attacking players are still at the World Cup with their national teams. Their team is now in the top eight. Hopefully none of them will suffer too serious injuries, and will integrate and perform well after the World Cup.