Manchester United crashed to a 3-2 defeat at home to Galatasaray on Tuesday to leave the Premier League side 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.
New striker Rasmus Hojlund was the lone bright spark at Old Trafford and twice gave the hosts 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 former United player 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 manager 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.
And 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.
Watch the video above, where Andy Mitten offers his take from inside Old Trafford after another disappointing night for United.