He described referee Damir Skomina’s late penalty decision, made after using the video assistant referee system, as a “disgrace” on social media.
Neymar was watching injured as United overturned a 2-0 first-leg deficit.
He said the penalty, which was scored by Marcus Rashford, “doesn’t exist”.
“It’s a disgrace. Four guys who know nothing about football watch a slow-motion replay in front of the television.”
The referee reviewed footage of the ball striking the hand of PSG defender Presnel Kimpembe before awarding the visitors an injury-time penalty that gave them a 3-1 win on the night, and saw them progress on the away goals rule.
“What can he do with his hand while his back is turned?” asked Neymar, who missed both legs through injury.