Manchester United crashed to a 3-2 defeat at home to Galatasaray on Tuesday to leave the Red Devils in danger of an early exit from the Champions League.
United have lost their opening two games of a Champions League group for the first time as new goalkeeper Andre Onana had another night to forget in Europe.
Rasmus Hojlund twice gave United the lead, but Galatasaray fought back to win away from home in the Champions League for the first time in 10 years thanks to goals from Wilfried Zaha, Kerem Akturkoglu and Mauro Icardi.
"We were totally in control and then we shot ourselves in the feet by giving goals away," said United boss Erik ten Hag.
"We have to defend in such moments and in certain areas better than we did."
Onana accepted responsibility after his error allowed Bayern Munich to open the scoring in United's 4-3 defeat in Germany to start Group A.
The Cameroonian was at fault once more for the crucial moment that saw Casemiro sent-off for conceding a penalty on 77 minutes.
Icardi missed the spot-kick, but quickly made amends to score the winner against 10 men.
United's 1-0 defeat at home to Crystal Palace on Saturday meant Ten Hag's men are also off to their worst ever start to a Premier League season with four defeats from seven games.
Progress to the last 16 of the Champions League now looks an uphill task.
Bayern beat FC Copenhagen 2-1 away to move onto six points, with Galatasaray four clear of United with four games to play.
- Hojlund shines -
Hojlund provided the one note of positivity on an otherwise costly night for United.
The young Danish striker's start to life at Old Trafford was hampered by a back injury, but he is quickly showing why United splashed out £64 million ($78 million) for a 20-year-old.
"He scored two brilliant goals," added Ten Hag. "I think especially the second where he was on his own and used his power and strength. It was a great finish."
Marcus Rashford played a huge role in the opening goal as his pinpoint volleyed cross gave Hojlund the simple task of heading in from close range.
The lead only lasted six minutes as Zaha came back to haunt his former club.
The Ivorian winger spent an unhappy six months at Old Trafford a decade ago either side of his successful spells at Crystal Palace.
Zaha had some fortune as his shot deflected off Diogo Dalot and over the hapless Onana.
"It was my first match in the Champions League, it was an explosion of emotions at that moment," said Zaha, who surprised many by turning his back on the Premier League to move to Turkey.
"What is important for me is to raise Galatasaray's name to the place it deserves - that is at the top."
United started the second-half with an intensity that has often been lacking this season.
Rashford should have picked out Fernandes for a tap in when clean through and Hojlund had a fine finish ruled out for offside.
United's centre-forward was not to be denied for long. He pounced on a slip by Davinson Sanchez and raced clear on cool before coolly chipping the ball over Fernando Muslera.
Galatasaray were immediately offered encouragement when Onana flapped at a harmless cross and Zaha teed up Akturkoglu who curled inches wide.
Akturkoglu was clinical moments later as the United defence was easily cut open by Baris Yilmaz's cross.
Worse was to come for Ten Hag and his new goalkeeper.
Onana was bought from Inter Milan for £47 million to replace David De Gea thanks largely to his ball-playing ability.
But a wayward pass went straight to Dries Mertens, who was chopped down inside the box by a desperate lunge from Casemiro.
The Brazilian was shown a second yellow card to leave United a man down for the final 20 minutes, including stoppage time.
Icardi briefly gave Onana a let off when he fluffed his penalty wide of the target.
However, the Argentine made amends on 81 minutes when Sanchez's headed clearance put Icardi clean through and he dinked the ball over the United goalkeeper.
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