Brendan Rodgers was left seething by a first half he said was played by an "Under-12 team" after Celtic were thrashed 5-0 by Paris Saint-Germain on Tuesday.
Beaten 7-0 away at Barcelona in their opening group fixture in last season's Champions League, the Glasgow side conceded three goals before the break at Parkhead against a rampant PSG.
Edinson Cavani scored twice while Neymar and Kylian Mbappe also found the net for the big-spending French club, with Celtic defender Mikael Lustig helping out the visitors with an own goal in the closing minutes.
While admitting PSG were impressive, Rodgers felt Celtic showed their big-name opponents too much respect, particularly in a one-sided opening half.
"This is a level where you have to look after the ball – we didn’t do that in the first half. We played like Under-12s. It’s confidence, it’s belief," he told reporters.
"We could have been better in the first half, we were too passive, didn't press well enough and stood off.
"In the first half-hour of the second half, we were much better but you can always concede against players of that quality. It's a phenomenal squad that Paris have but I was proud of the players in the second half and the supporters were terrific tonight, they kept encouraging us.
"At this level you have to make the ball work, and you have to have that belief. In that opening period, there was too much space, the physicality wasn't what it should have been – whoever we were playing, it wasn't what we would like.
"It's a tough lesson, a harsh lesson for us. This is a level which is all about learning for us, and tonight we'll learn from that."
The 5-0 defeat is Celtic's heaviest in European competition, although Rodgers saw enough in the second half to suggest his side can still compete in a group that also includes Anderlecht and Bayern Munich.
"I'm not worried about records. Tonight we were playing against world-class opponents, but in the second half – it's always easy to play at 0-3, I want them to start like that," the former Liverpool manager added.
"If we get better we can achieve the main aim, which is European football after Christmas."