Manchester United boss Ole Gunnar Solskjaer admits losing to Istanbul Basaksehir away from home cost his side in the group stages.
The Red Devils fell short after a 3-2 defeat to RB Leipzig on Tuesday.
It is a result that means they will play the rest of the season in the Europa League knockout stages.
While two losses in the last two games to PSG and Leipzig hurt their cause, Solskjaer believes losing to the inferior Basaksehir was the biggest mistake his team made.
"We weren't good enough," Solskjaer told BT Sport.
"We started really well, but obviously the big turning point or the big defeat for us was the one away to Istanbul. That was the one that you look back at and think that is where we lost the points which we should have had.
"Today we were close, but we knew we had to defend crosses, knew we had to defend balls in the box and we just couldn't clear them."
