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It's very natural that after losing Lionel Messi, Barcelona seemed to have lost a leader on the field and that was clearly felt. Even other players such as Sergio Bosquest and Jordi Alba also could not restore the confidence of every player. I think barcelona should have prepared for that and there's no way they will continue to rely on just one player. On the other hand, they also have to develop more and with a new squad, all the legacy of their seniors must continue with the same mentality. Maybe it's not appropriate for a team like Barcelona to experience this continuously and I think they have to break this bad result in the Champions League this season.
Barcelona have gotten used to being without Messi even though the fans miss him. Barcelona lost a lot of quality from their game not only the Messi factor, but they don't have a lot of money to build a quality squad. So far Barcelona has not lost character, but it is true that they are not as good as before in the Champions League.
I believe that if Barcelona is in good financial condition, then Xavi will easily realize his dream of winning the Champions League title. There are many top players who are willing to play for Barcelona, but because Barcelona's financial situation is bad, they will not take a place.
Mind you, Barcelona had Messi from when he was small, hence the reason why the team seemed to have been built around him. It is way easier to train a star player from small. He is more like family until he has gotten to the peak with them when he may not be able to perform as much, that's why he was let off too.
Right now, any start player they have will have to have been groomed from the youth team, do as to fit well within the managers plan for the season.