The shirt became vacant after Adama Traore's loan at Barca expired. In the past, players like Neymar and Ousmane Dembele wore it.
In his first season at Barca, Ferran wore the no.19 jersey. Back when he was signed, that was the only available shirt except for no.25, which is usually reserved for goalkeepers.
Source: La Liga (h/t Barca_Buzz @ Twitter)
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