In a throwback to the era of Istanbul's infamous old Ali Sami Yen Stadium, where United laboured in Europe 30 years ago, another banner read 'Your Nightmare Is Back Again' - and they were prophetic words for the hapless Onana.
If United fail to make it out of Champions League Group A, and there is every chance that happens after they twice squandered a two-goal advantage to draw 3-3 in a chaotic classic in Istanbul, Onana will take a large share of the blame.
Bayern Munich's draw with Copenhagen later in the evening means United must beat the German side in a fortnight and hope Galatasaray draw with the Danish side in order to qualify.
Erik ten Hag says Man Utd throwing away leads is 'my responsibility'
United's inability to cash in when in control of games in the Champions League this season has been an Achilles heel.
They also let a 2-0 lead slip in the 4-3 loss in Copenhagen and could now face a heavy price, with their fate out of their hands and Bayern Munich, with of course Harry Kane, to come in the final game at Old Trafford.
No-one can complain about entertainment value in United's return to the Champions League but it rather loses its lustre when you list the five leads they have lost.
Fans, management and players will look back with regret at a 4-3 loss away to Bayern, a 3-2 defeat at home to Galatasaray, that 4-3 reverse away to Copenhagen and now this magnificent rollercoaster in Istanbul.
All very entertaining - but not necessarily for the Manchester United fans who would simply prefer to see their team win.
And if United are to be consigned to the Europa League on a Thursday night, or even no European football at all after Christmas, then a lot of the blame will be directed at Onana's performance against Galatasaray.
His performance in Turkey once again flouted his billing as one of the game's top keepers when he arrived from Inter Milan in the summer after helping the Italian club reach last season's Champions League final.
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Reaction to United's draw in Istanbul
It would be stretching the point to suggest tears were shed when David de Gea ended his 12-year association with Manchester United last summer, but there may be some wistful glances towards a goalkeeper still seeking employment given Onana's performances so far this season.
The Cameroon keeper has looked uncertain and vulnerable - although not necessarily without confidence, judging by one neat pirouette almost on the halfway line against Galatasaray - and has not yet resembled a vast upgrade on De Gea.
Since 2018-19, he has made the most errors leading to opposition goals by goalkeepers in the Champions League, with seven costly mistakes.
If Galatasaray's fans wanted to conjure up images of nightmares, it certainly worked for Onana, who produced a horror show to undermine much of United's good work.
United looked as impressive as at any time this season when they burst out of the blocks, unaffected by the usual deafening atmosphere to strike two wonderful early blows from Alejandro Garnacho and captain Bruno Fernandes.
It was all going as smoothly as things can go against Galatasaray on their own turf before Onana got his positioning all wrong for Hakim Ziyech's free-kick that put them back in the game.
Scott McTominay restored United's position of relative dominance by making it 3-1 10 minutes after half-time until Onana conjured up a truly rotten piece of goalkeeping to give the Turkish champions hope once more just after the hour.
Ziyech's angled free-kick with his left foot hardly carried mighty power and was directed straight at the goalkeeper with no other players in close attendance applying pressure.
Onana appeared to attempt to scoop the ball out with his right hand only to oblige Galatasaray by helping it into the net. Ziyech looked almost as stunned by the outcome as United's keeper and his team-mates.
It gave Galatasaray momentum just when it seemed they were starting to run on empty and they earned their point with a powerful strike from substitute Kerem Akturkoglu with 19 minutes left.
There followed a frenetic finale with chances at both ends, a winner somehow avoiding either side as the game became wide open in the closing stages.
The 27-year-old arrived in a £47m deal with a big reputation and as someone well known to Manchester United manager Erik ten Hag from their time at Ajax - but he has done little to justify his fee and this was his worst night so far.
It was an even bigger source of regret because much of United's play was commendable until Onana took a hand - or in this case, did not.
United must now beat formidable Bayern Munich at Old Trafford on 12 December, although they may be helped by the fact Thomas Tuchel's side have already topped the group.
If United fail to progress they will be desperately frustrated after a campaign that has contained so many self-inflicted wounds. This jeopardy could have been avoided.
There is still hope but
