Brendan Rodgers felt Celtic started well against Barcelona, but blamed fatigue for what he described as a "bitterly disappointing" Champions League loss.
A hat-trick from Lionel Messi, two goals from Luis Suarez and a strike apiece from Neymar and Andres Iniesta saw the Spanish champions inflict upon Celtic their heaviest defeat in the history of the competition.
Messi gave the home side the lead after just three minutes at Camp Nou and Moussa Dembele spurned the chance to level matters at 1-1 when his penalty was saved by Marc-Andre ter Stegen.
Rodgers acknowledged Dembele's miss was a pivotal moment and was hugely disappointed with the final score.
"Of course, it's never nice professionally when that happens," he said. "I thought in the first period of the game our set-up and organisation was good.
"We gave away a goal very early which is certainly not what you want to do here. But I thought we came back into the game and once we actually got a bit of belief and a bit of confidence to pass the ball then we started to open up and create opportunities.
"It was always going to be really tough to play the best possession team in the world after a hard game at the weekend [against Rangers].
"Obviously there's a massive moment in the first half. If you can score the penalty and get it to 1-1 then it can make the crowd a bit nervy, especially after the weekend loss [against Alaves]. To then very quickly go down to 2-0 was disappointing.
"Then second half, I think we ran out of legs and the scoreline by the end is bitterly disappointing."