Barca don't need to have the same performance of last season,if not they might not be able to win the next champions league. Barca needs to improve in their performance so that they can be hopeful to win UCL. The champions league is a competition that brings the best clubs from different leagues to play amongst themselves and more is expected from most clubs. Winning your local league hasn't given you a guarantee that with the same performance you will win a competition played by the best clubs in Europe. I still believed that with the little change in the players Barca will do perfectly well in the champions league this season. As they will want people to believe in them by impressing their fans with good results.
Barcelona's performance in the Champions League last season was disrupted because several players were injured. Their defense was a mess and it looked very different when they played for La Liga. Xavi tends to trust Alonso as a left-back compared to Balde or Jordi Alba, it's also a form of mistake because Alonso is not in good form as a newcomer.
I hope Xavi can really learn from Barcelona's mistakes and his strategy in the last two seasons in the Champions League, so he should have a good plan for next season's Champions League. But still, I don't think Barcelona has a big chance to win the Champions League title soon.