Neymar has lost the "direct-ness" in his play that he acquired after his first season with Barca. That's what made him one of the best in the world, as he had finally added substance to his flashy style. He seems to have regressed at PSG and resorted to his old self. It doesn't help that he's A) clearly still not a 100% following his lengthy injury and is struggling particularly with his explosiveness, and B) is trying to do way too much. The luxury Neymar has in Brazil that Ronaldo and Messi don't have with their respective national teams is, he doesn't have to carry the team. He doesn't have to always make something happen. He can just fit into the system, play well, and they'd have a great chance at winning.
That said, I think if Marcelo's WC is over, in addition to Douglas Costa's injury (he's out for the rest of the tournament), Brazil may struggle.