Barcelona might need a quite long time to bring back their old days in the Champions League. Because I don't feel like it is only related to Xavi's strategies now. Barcelona are rebuilding with very young players mostly. This team will need some more time to be able to reach the level at which they can fight harder against the biggest favourites.
Hence I won't be expecting them to have a magical change this season as well. Maybe this time they would manage to complete the group stage successfully. But I don't think they still have a strong mentality to deal with big teams in later stages. This tournament is nothing like a domestic league.
Barcelona can have improvement in a new season by going through the group stage but at knock-out phase, they will face with stronger team and with more experienced squad. It will become big issue for Barcelona with their young players and the young coach like Xavi. They need more experience in Champions League to play well and win it but everything needs a start and a new season will help them to gain more experience.
The last two seasons have been a nightmare for big teams like Barcelona and they probably don't want that to happen again in the Champions League next season. As a coach Xavi is fully responsible for the performance of the team he has built and as long as they can still prove their capacity then they will probably be much better later.
It is not about what they want or don't want, it is about their ability to play well and achieve good results. Barcelona have been playing worse in Champions League after losing Messi. But previous seasons, when they had Messi, all responsibilities were blamed on Messi unfairly. Now they have to pay the cost and accept the fact that their squad is not actually good and competitive, with or without Messi.