UEFA today confirmed they are launching an investigation into Neymar's comments via a statement on their website.
Brazil global Neymar watched from the stands at Parc des Princes as United claimed a 3-1 win in the second leg of their last-16 tie to progress to the quarter-finals on away goals.
Neymar, 27, was incensed with Damir Skomina, the referee, for awarding United an injury-time penalty when Diogo Dalot's shot struck Presnel Kimpembe's shot.
Marcus Rashford's strike sent PSG packed.
"It's a disgrace", Neymar wrote on his social media account.
UEFA says it "appointed an ethics and disciplinary Inspector to conduct an investigation" of the social media comments.
"How do you handball behind your back?"
Neymar, meanwhile, has not featured for PSG since he went off injured in the side's 2-0 victory over Strasbourg on January 23.
Neymar watched last Wednesday's match from the stands, having returned from Brazil where he had been recovering from a second fractured fifth metatarsal in his right foot.