Celtic skipper Scott Brown described the second half of their 7-0 thrashing by Barcelona at the Nou Camp as unacceptable.
The Hoops were losing 2-0 at the interval through a Lionel Messi double, but Moussa Dembele had a penalty saved by Marc-Andre ter Stegen when it was 1-0 for the home side.
However, the Scottish champions capitulated after the break and Messi added a third for his hat-trick, Luis Suarez a double with further goals from Neymar and substitute Andres Iniesta.
It was Celtic's heaviest European defeat and Brown was not happy.
"Everyone in that changing room was disappointed. It was a heavy, heavy defeat," Brown said. "I think in the first half we had a penalty and I know we miss it but that's football, but it is the second half that is really disappointing.
"To lose five goals in 45 minutes is unacceptable. We didn't do well enough on the ball, they kept giving it back and they are entitled to score, they are a great team. But we should do better than that.
"It is hard to take but yet again, they are a great team but we need to bounce back. We have Man City the next game, and we need to take something at Celtic Park, and with our fans anything can happen."