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.
That is not the only reason the reason is that the team wasnt ready, it was too little time playing with Xavis schemes and all together as a real team, the defeat was inevitable in my view
now after a year together, having won La Liga, they are now motivated to move forward. And this year go deep in UCL.