Barca were the favorites but Eric Garcia got a red card in the 10th minute and they had to play with 10 men for a very long time. Nevertheless, they scored the equalizer and had chances to win the game but couldn't capitalize. It was an unfortunate result for Barca but we can't blame them too much. The only criticism we can make is that Flick didn't make any changes until the 2nd goal they conceded. He could have intervened earlier in the game.
Away games will be difficult in the new format of the Champions League. If they don't make mistakes in the home games, Barca will still have a top 8 chance. It's bad to start with a defeat, but we have seen how this team plays in La Liga. They will get better.
It was the same thing that happened last season against PSG when Barcelona was winning 1-0 and a red card made them scared because of weak mentality that they can still win the match if they don't play with fear but with hope of winning PSG. Look at it, the same thing has happened to them the season and they still lost the match.
I think the coach should always prepare them for a man down mindset so that they can know how to play the match and draw it than losing it because it is possible.