Ole Gunnar Solskjaer said his team's defensive performance was 'unforgivable' after losing 2-1 to Istanbul Basaksehir on Wednesday.
"The performance wasn't good enough against a team that worked and ran, and they beat us on the break a couple of times, and scored two goals like you do sometimes in Europe, when you don't defend well enough, " he said after the game.
"The first one is we play a short corner, and we forget about the man up top, and that is unforgivable.
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"The second one as well, we're not very well organised to counter press and win the ball back. Two soft goals to give away but we didn't have enough to come back.
"We scored from a cross, a good goal from Anthony. Then you think, 45 minutes to save something from the game. We threw everything at it but we couldn't break them down."
After the 1-0 defeat to Arsenal on Sunday in the Premier League, the Norwegian admitted it was another blow to his side's confidence after an awful start to their campaign.
"It has set us back a lot. You get a reality check every time you play a game of football. Nothing becomes history quicker than a game of football," he explained.
"You don't just turn up and get three points in the Champions League. We weren't good enough.
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"It's not easy to be positive when you've just lost the way we did. These players are good players, and we have a good chance on Saturday morning to get going again."
Even when ending on a positive note, Solskjaer admitted his side had underperformed again.
"It set us back a lot. You get a reality check every time we play a game of football here. Nothing becomes history quicker than a game of football. You don’t just turn up and get three points in the Champions League. We weren’t good enough," he said.
"It’s not easy to be positive when you’ve just lost the way we did. These players are good players and we have a good chance on Saturday morning to get going again."