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It's just a loan, so Barcelona didn't lose anything if Ansu Fati can perform and regain his confidence back. Maybe De Zerbi willing to pay big amount of money to sign Ansu Fati, but it's an another discussion in the future.
Barcelona also don't have any problem with their squad, Yamal is replacing Dembele and Felix is replacing Ansu Fati.
Ansu Fati departure did not have too much of an impact on the Barcelona squad as they got a replacement straight away before the transfer window closed. The arrival of Joao Felix will be a replacement for Ansu Fati, Xavi Hernandez already has a complete squad after successfully bringing in new players in the last seconds of the transfer window.
Ansu Fati was not released permanently by Barcelona, he was only loaned to Brighton to get more playing minutes. Of course there will be further discussions if Brighton wants to sign Ansu Fati permanently, Barcelona has also put a fairly high release clause in Ansu Fati contract which means Brighton has to spend a large amount of money to make Fati permanent.
Yamal's presence meant that Xavi had to choose between Yamal and Raphinha to fill the main line up. Yamal potential is quite good, he needs to gain the coach's trust to hone his skills, while Raphinha is very experienced in the senior team. Xavi had to rotate between the two, the position left by Dembele didn't really bother Xavi about the situation in the first team.