Xavi was able to finish the season well in La Liga, but he failed to do so in the Champions League. Perhaps he has little experience in such competitions so far and it is easier for him to focus on one thing at a time. I think that in the current Champions League season Barcelona will not achieve great results again, because the competition is very big.
The barcleona's squad was not depth enough last season. That's why xavi has been buying some talented players like guendogan from manchester city as he wants barcelona will be fully prepared to face UCL this season.
The competition becomes even stricter and manchester city as the winner of last season has been doing massive improvements. Even city has bought the three times UCL winner kovacic from chelsea as his replacement. Barcelona were unable to get a good result in europa league last season. It has been proving if barcelona needs to strengthen its squad. The defensive line must become the main concern by xavi. Lewandowski is good in helping barcelona