Marcus Rashford was sent off on his return from injury as United conceded twice in the final ten minutes despite taking the lead while down to ten men to leave them with a mountain to climb.
A trip to the hostile atmosphere Galatasaray awaits later this month and defeat there could knock United out, piling even more pressure on ten Hag.
United were certainly not without their positives but somehow contriving to a costly defeat sums up the chaotic nature of their season. Rasmus Hojlund scored twice to move alongside Alvaro Morata as the competition’s top scorer with five goals in his four games, while United’s performance when down to ten was positive before the late collapse.
"It will turn. It will turn. There will be one moment in our season,” said the former Ajax manager.
“I take a lot of positives. Even with 10 men, we are dictating the game."
On Rashford’s sending off, ten Hag added: “When you freeze it, it makes it look so worse.
"The game is never meant to be like this, this has nothing to do with football.
"Decisions have to be made and I accept. Wrong decisions are made."