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.
Brown said: “Everyone in that changing room was disappointed. It was a heavy, heavy defeat. In the first-half we had a penalty and I know we missed 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.”
The Parkhead midfielder is looking to take some lessons from the Barca defeat, put it on the back burner and and get back on track in the league against Inverness on Sunday.
He said: “They are the best team in the world, there is no doubt in that. But we need to defend better from the front middle and back, be more compact and brave on the ball and even here we usually do better than that.
“We have another big game on Sunday against Inverness and we need to keep our run in the league going, keep our 100% run going and we will focus on the Champions League when it comes around again.”
Boss Brendan Rodgers insists Celtic’s season will not be derailed by the 7-0 thrashing.
It was the Parkhead club’s heaviest European defeat but with the Hoops top of the Ladbrokes Premiership with four wins out of four and in the Betfred Cup quarter-finals, Rodgers said: “It doesn’t matter if you are playing Barcelona or Berwick Rangers, there is professional pride and no professional likes to lose like that, even though we are playing against a team of top, world class players.
“But you can’t be too downhearted. We have had a brilliant start to the season and this won’t hamper us in any way, you will always take the positives from it.
“It is tough when you lose so many goals. But these players have been brilliant.
“They could not have come to a more difficult place in world football, after a highly intense game at the weekend where they put so much into it and dealt with so much pressure and played so well.
“To have hardly any preparation time to come and play against the world’s best at keeping the ball and tiring you out, and beating you up with the football at times, it was a tough ask for them and that last 15/20 minutes did feel long.
“Even the older ones will learn from that and that will make us a better team.”