And based on the last four months, that is a task that seems to be beyond Onana. The Cameroonian’s short career with United has been a traumatic experience for everyone involved, beginning with him getting lobbed from the halfway line on his Old Trafford debut in a friendly against Lens.

But Champions League nights have been particularly harrowing for the £47-million ($59m) signing, and his disaster-class in the 3-3 draw at Galatasaray, which leaves United bottom of Group A with one game to play, was the worst of the lot.

For United to reach the last 16, they now need to beat Bayern Munich in their final group game and hope that Galatasaray and Copenhagen’s match ends in a draw. In other words, they need a miracle. And if they cannot pull it off, Onana will be the biggest culprit for their return to Europe’s elite competition being short-lived.

Only Antwerp have conceded more

Onana had a rough start to life in the Premier League, conceding 16 goals in his first 10 matches before keeping three clean sheets in a row. But his numbers in the Champions League have made for even grimmer reading. In just five European matches for United, the Cameroonian has kept just one clean sheet while conceding 14 goals.

That gives United the second-worst defensive record in the Champions League, only trailing Royal Antwerp, who have let in 15. The Belgian champions can be cut some slack as they are playing in the competition for the first time since 1957, but United, as three-times European Cup winners, have much higher standards. And Onana is simply not meeting them.

Big errors in three matches

While United’s outfield players may not have helped Onana out by failing to see games out and squandering the lead on three occasions, the goalkeeper has been the architect of their downfall more often than not. Onana has made standout errors in three games in particular, of which United ended up taking just one point out of a possible nine.

His traumatic campaign began with him letting Leroy Sane’s tame shot squirm under his body to give Bayern Munich a 1-0 lead in their eventual 4-3 win in Bavaria. Against Galatasaray at Old Trafford, meanwhile, with the game tied at 2-2, he made a careless floor pass which was scooped up by Dries Mertens. His mistake forced Casemiro to make a last-ditch tackle, leading to him conceding a penalty and getting sent off. Mauro Icardi missed the target from the spot, but Galatasaray eventually made their numerical superiority count and the Argentine scored a few minutes later to clinch a 3-2 win.

Twin gaffes in Istanbul

Onana’s terrible run against the Turkish giants continued in Istanbul, when he failed to organise his wall properly for Hakim Ziyech’s first free-kick and got his positioning badly wrong to concede the goal which halved United’s advantage. But that error paled in comparison to the gaffe he made in the second half from another Ziyech free-kick.

This time Onana didn’t even bother to assemble a wall, which invited the Morocco international to shoot. The strike was tame and straight at the goalkeeper, and most of Onana’s counterparts would have caught it comfortably, but the Cameroonian contrived to parry the ball to his right and inexplicably ended up punching it in the other direction and into his own net, making it 3-2.

There was little Onana could do about Kerem Akturkoglu’s thunderous equaliser, but he had already given Galatasaray two footholds in a game that United looked to have had in the bag.

 ImagesSlow reactions & poor positioning

Those were the cut-and-dry moments, but Onana has also been criticised for his slow reactions and poor positioning for several of the other goals United have conceded in their harrowing Champions League campaign.

He was too slow to come out and close down Wilfried Zaha’s shooting angles when the former Crystal Palace forward struck Galatasaray’s first goal at Old Trafford, while he allowed himself to be sat down by Icardi for the winning goal. Had he stayed on his feet, he would have saved it.

In the chaotic 4-3 defeat at Copenhagen, he could also have done better for three of the Danish side’s goals. He was too slow to get down for Mohamed Elyounoussi’s strike and was far too passive when Lukas Lerarger slipped in front of him to score the equaliser, which bounced off Onana’s body and into the net.

Roony Bardghji’s winner was struck with far more venom, but Onana could still be criticised for being beaten at his near post.

Like an outfield player forced to go in goal

The one standout moment for Onana in the Champions League was his last-gasp penalty save from Copenhagen’s Jordan Larsson which preserved the Red Devils’ 1-0 advantage and gave them their only win in five matches.

Along with goalscorer Harry Maguire, Onana was United’s other highly unlikely saviour that night. But while the defender used that moment as a springboard for a remarkable revival, Onana has reverted to type and continued to look like a wholly unconvincing goalkeeper.

Indeed, his incompetence for basic goalkeeping skills and his aptitude for passing and dribbling means he often looks like an outfield player who has been forced to go in goal as a team-mate has been sent off. He does not seem able to command his box and whenever he is one-on-one with a striker, he always looks as if he is going to come off second best.

While most goalkeepers come flying forward to narrow down the angles and try to intimidate the onrushing forward, Onana often retreats, giving his opponent more time to pick his spot. That was the case when Zaha scored at Old Trafford and when Taiwo Awoniyi came towards him and met little resistance before scoring in the second minute of Nottingham Forest’s trip to the Theatre of Dreams.

 ImagesAn expensive mistake for Ten Hag to fix

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.