Their problem is that they haven't found Pique's replacement. Pique is already 35 years old and his physique has declined. Even Pique always has a hard time facing a striker who can run quickly. I think Xavi should start thinking about the central defender. I think he should be brave to start the rotation,for example by installing Kounde as the starting line up. Because Xavi is carrying a mission of improvement and he deserves to give more playing time to the younger defenders.
Playing with young defenders are always a big gamble, all other positions you can fill youngsters but especially central defenders need to be experienced. Major clubs like Barcelona have no luxury to play with inexperienced defenders.
I don't think Barcelona had any chance left in Champions League anyway. They should go to Europa League route and try to win that trophy.
you are right Barcelona's current defense lineup is in a very poor state. Adding some new defense to their team is now essential. Also I would like to see a change in the midfielders, who should respect Busquets and if we add a good midfielder in his place the team will become stronger.