The shocking error sparked outrage among Red Devils fans on social media, while supporters of other clubs mocked Ruben Amorim's team.
"Sell Onana immediately," wrote @zayn4pf, while @Lea_EFC said: "Dorgu with his first assist for Manchester United. Where the f*ck is Onana going."
Journalist Samuel Luckhurst wrote: "Disastrous from Dorgu. Onana is clapping him. Breakdown in communication but Dorgu has to see Onana coming."
Meanwhile, @wealth_united wrote: Getting rid of De Gea for Onana was the biggest mistake we ever did as a club. We can't keep on pretending."
"Why is Andre Onana still playing for Manchester United?" asked @JoeSillett. "He's an absolute liability. He is a terrible terrible goalkeeper. He has no confidence, doesn't communicate with his defence and should never ever play for Manchester United again."
The defender was the main culprit in @_Utdbaki's eyes, who posted: "Onana had it covered, but Dorgu wasn't concentrating. He played the pass thinking Onana was still in goal. He should have guided the ball until Onana claimed it."