I know that Barcelona played the worst match of the season. But you are underestimating the mental factors in this game very much. If Barcelona got ahead then I think there would be a different game than we saw in the field. As a result this match is left behind and the thing Barcelona can do now is to learn their lesson from this match and focus on their upcoming matches.
Well psychological effect always play it role but I still don't think they will have a easy win like other games compared to that particular game at most I still think a draw or maybe a narrow escape of 1 -0(which is of low probability imo)... I don't know if you noticed a decline from th last UCL, I did said it was a bosting one because the game performance was upto top notch compared how they've been doing it before and their last game exposed them more .. I believe they should have enough time to rest this player they are somehow tired with the way their playing. Thus, With 3 point down they will want to step up their games the more
Losing three points is a huge problem for Barcelona. That's why I'm sure they will focus more on other matches, the fact that they didn't shoot as well as their opponents in the match they lost requires them to be careful about this. Since they are also targeting the Champions League, they have to rotate their squad in both the Champions League and the Spanish league. That's why the harmony between the players is so important. Hansi Flick should be able to provide this harmony. The players coming from La Masia are saving Barcelona right now.