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.
You can always find an excuse for bad results, but in my opinion, the financial crisis did not prevent Barcelona from becoming La Liga champions. What has changed this season, and who do you think should be sold? Xavi has already said that they can sell Lewandowski, I understand what he is talking about, Barcelona has problems in attack, but perhaps the reason is not only Lewandowski, maybe the strategy that Xavi chooses also matters? Because he puts a lot of emphasis on defense, and in this case it’s difficult to expect the team to score a lot.
More importantly than which player Barcelona will sell, Barcelona need an experienced striker. Also this season we have seen poor performances from Barcelona's defenders. Barcelona's defense was relatively weak. Barcelona's defenders did not play well, especially against bigger and stronger teams. On the other hand, Barcelona have been reliant on Lewandowski to score this season too.
Barcelona need experienced striker and talented defender now. Barcelona's management will have to spend a lot of money to strengthen the defense and attack. And because of financial problems Barcelona team is not able to strengthen their squad.