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.
There is always a risk when placing young players in the line-up, especially the central defender position. That's a crucial position for defense that must be filled by players who have experience, composure and can read the direction of the ball. It's hard to find a centre-back with that kind of quality. But I agree that player rotation can be one way before the transfer market opens. However, we cannot know the quality of a player without giving him a chance to play. It is very risky indeed, but at least there is an effort to find players who can fill these positions.