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.
I don't think Kounde is too young a defender, he must have had quite a bit of experience at Sevilla before. Then Eric Garcia isn't a young defender either, but he might not be the best if Kounde and Araujo are fit to play the game.
Pique is not young anymore, then Busquets is the same. But Xavi hopes they can give Barcelona peace of mind so that the team's performance is not chaotic. But it didn't stop there, they both made many mistakes that made their team lose. Not blaming them, but Xavi should be brave enough to put them on the bench if they don't perform well.