Onana's Champions League nightmare with United is all the more remarkable considering how he was one of the best goalkeepers in the competition last season, helping Inter reach the final. Pep Guardiola lavished praise on him on the eve of the final and he gave an accomplished display in Istanbul, looking better with his feet on the night than his opposite number Ederson.
With United on the hunt for a replacement for De Gea and given Onana's years of experience playing for Ten Hag at Ajax, he seemed like an obvious signing, and Inter's £47m asking price seemed perfectly reasonable.
But just six months after the Champions League final, Onana now looks like a huge waste of money for United. He has turned into a liability and an easy target for opposing teams, who feel like they have a decent chance of scoring even with pedestrian shots.
The bad news is, they do not have many alternatives. The club signed Turkey international Altay Bayindir as back up in the summer, but he left his previous club Fenerbahce by the back door, with fans turning on him as his form began to drop off.
And it seems that Ten Hag does not have much faith in the 25-year-old as he picked Onana ahead of Bayindir for both matches in the Carabao Cup and still preferred to select the Cameroonian at Everton last Sunday even though he had picked up an injury while on international duty.
Saturday's trip to Newcastle, who thumped Chelsea 4-1 last week and will be extra fired up after their controversial draw with Paris Saint-Germain, seems like a difficult scenario to give Bayindir his introduction to English football. But persisting with Onana, who frequently looks like a disaster waiting to happen, is equally risky.
The Cameroonian was supposed to solve a long-running problem for United and bring them in line with rivals Liverpool and Manchester City, who long ago realised the need for a goalkeeper who knows how to distribute the ball well. Instead they appear stuck with someone who is lacking the basics of the art of goalkeeping, and rather than providing assurance between the posts, seems to drain confidence from his team-mates while giving hope to United's opponents.
In Istanbul, United were not just left confronting the financial hit of their likely elimination from the Champions League. They were also left scratching their heads over what to do with a goalkeeper who is failing to perform his principal task of stopping their opponents from scoring.