Ole Gunnar Solskjaer admitted it was "not easy to be positive" after "unforgivable" defensive mistakes saw Manchester United lose to Istanbul Basaksehir.
The Red Devils, who had so impressively beaten Paris Saint-Germain and RB Leipzig in their first two UEFA Champions League Group H games, saw an 18-game unbeaten run away from home end in a 2-1 defeat on Thursday (AEDT).
Basaksehir took the lead when 35-year-old striker Demba Ba was allowed a free run at Dean Henderson's goal from inside his own half after a simple clearance following a United corner routine.
It was 2-0 when Edin Visca struck after Juan Mata had lost possession in his own half, and although Anthony Martial pulled a goal back quickly, United's dominance of the ball yielded just one further shot on target from a Bruno Fernandes free-kick.
Former United midfielder Paul Scholes compared the defending for the opener, where nine United players were in or around the penalty area and Nemanja Matic had allowed Ba to drift clear, to "Under-10s football", adding on BT Sport: "What the back four are doing, I have no idea."
Ex-teammate Rio Ferdinand tweeted his frustration with United's defending, saying: "The goals against us... please tell me someone is going nuts in that changing room and people are being made accountable."
Solskjaer accepted there was little to praise about his side's performance, which followed a 1-0 home defeat to Arsenal in the Premier League last Monday.
"The performance wasn't good enough against a team that ran and worked and caught us on the break," he said, as per United's website.
"We didn't defend well enough. The first goal, we play a short corner and we forget about the man up top, and that's unforgivable.
"The second one, we lose the ball and we're not very organised enough to counter-press. So, two soft goals to give away and we didn't have enough to come back in the second half.
"We scored a good goal by Anthony, and then you think, 'Forty-five minutes to save something'. We threw everything at it but we couldn't break them down.
"It set us back a lot. You get a reality check when you play a football game here. You don't just turn up and get three points in the Champions League. We played against lots of good individuals in a good team, well set up and organised. We weren't good enough.
"It's not easy to be positive, not when you play like this."