I have mixed feelings about Messi's injury and the reality that he will not feature in the second match against Benfica. Although I must confess that it is scary knowing that it is just a month to the World Cup and I wouldn't want any injuries to prevent him from playing, on the other hand if there is anything I have learned from studying Messi is that he always bounces back from injury in exceptional form.
Judging by the comments of Galtier, there is nothing serious there and Messi misses the game with Benfica simply for preventive reasons. By the game with Marseille, he will already be in the squad. I don't think that the absence of Messi will prevent PSG from achieving the desired result in the home game. Even if Benfica puts a bus at their gates, Mbappe and Neymar will find a couple of necessary loopholes. But if, by some miracle, Benfica takes away points from PSG, then the situation in the group will become as interesting as possible, since in this case Juventus will regain their chances to qualify.