The PSG star took to Instagram to hit back at the decision against his team following a handball VAR review during their Champions League last-16 clash.
The referee awarded a penalty to the Red Devils for handball by Presnel Kimpembe in the final moments of the game, and Marcus Rashford converted the spot-kick to eliminate the Ligue 1 champions from the competition.
Neymar, who did not play in the fixture due to his current foot injury, voiced his opinion on social media after the match.
"That is a disgrace," he wrote on his Instagram Stories alongside a screenshot of the alleged handball.
"And UEFA still pick four guys who know nothing about football to review the VAR decision in slow motion... that handball simply doesn't exist!
"How do you handball behind your back? Ahh.. go f*ck yourselves."
Following the clash, UEFA announced they would be looking into the 27-year-old's post-match comments and have opened an investigation into the way he conducted himself online.
A UEFA statement read: "In accordance with the Article 31 (3) (a) of the UEFA Disciplinary Regulations, UEFA has today announced that it has appointed an Ethics and Disciplinary Inspector to conduct an investigation regarding the comments made on social media by the Paris Saint-Germain player Neymar Junior, following the 2018-19 UEFA Champions League match between Paris Saint-Germain and Manchester United FC, played on 6 March 2019 in France."
The club's latest Champions League elimination was their third consecutive defeat at the last-16 stage.
They have failed to advance to the last eight of the competition since the 2015-16 season, with the club still having never reached semi-final stage in their history.
Neymar, meanwhile, has not featured for PSG since he went off injured in the side's 2-0 victory over Strasbourg on January 23.