Actually, and I also think this is the best solution for Barcelona, they should just sell their players who they know are not doing well for them again, are old enough, and can participate with their full strength in the full season. You have a point. It always takes time when the league starts, and players who are not in good shape or are old may seriously deform. If there is no other player to replace, then there will be a problem, so to me, the best thing is to get new players with good performances. Sometimes experience does not only matter; strength also needs to be involved in football.
Barcelona should definitely sell players who are not doing well with the squad. But the problem is that Barcelona do not have enough funds. Barcelona will need to buy new players if they sell old players. Barcelona are unable to add new talented players to the squad due to insufficient funds. Perhaps this is the reason why Barcelona management is not deciding to sell old players.
The only reason Barcelona is in such bad shape is the financial crisis. Xavi could have built a stronger squad if not for the financial crisis and would have been able to strengthen the Barcelona team as well. Xavi is failing to stabilize this team only because of financial crisis.
I would never say that Xavi has completely failed at managing his players so far. Barcelona have shown significant improvement under him, but losing several key players in the same season is very difficult. The financial crisis experienced by Barcelona has certainly slowed down Xavi in building his best squad, but if Xavi and management can be patient, then the academy players who are currently carrying heavy responsibilities will prove their quality.
Several main players may be sold by Barcelona in the upcoming transfer market, and even several players who are currently on loan are expected to be made permanent by the team that uses their services. Eric García, Ansu Fati and several other players may be made permanent by the team that provides them at the moment, but it is not known what Barcelona wants.