The Brazil worldwide blasted the use of VAR to award Manchester United a late penalty in their 3-1 Champions League round-of-16 second-leg win at Parc des Princes last week that secured a 3-3 away goals success.
"This is a disgrace", Neymar wrote on Instagram shortly after the match.
Neymar said the penalty "doesn't exist".
UEFA says it "appointed an ethics and disciplinary Inspector to conduct an investigation" of the social media comments.
The 27-year-old Neymar was so reportedly enraged he had to be restrained by PSG staff to stop him entering their dressing room. How can it be a handball when it hits...
Buffon accused the English referee of having a "dustbin in place of a heart" for awarding Real a last-gasp penalty that knocked Juve out after the Italians had clawed back a 3-0 first-leg deficit to level the tie.
Neymar, who did not play in the fixture due to his current foot injury, voiced his opinion on social media after the match.