Barcelona has several reliable defenders, Jules Kounde, Ronald Araujo, Andreas Christensen. it's just that, they got injured and the worst one was Araujo. some veteran players, can be an option against teams that are not too strong. in midfield, Gavi and Predi, really still need high flying hours until their potential is honed. these two young midfielders, often lose their game during crucial matches. Sergio Busquets, should have been replaced. he is no longer able to play with his best performance.
Araujo will recover soon, meanwhile Garcia Kounde, Christensen and Alonso could be good options at centre-back. Xavi has a lot of players at the back right now, but it's still difficult to find the right one at right-back even though they currently have Bellerin.
In midfield, I think Busquets is still good enough to captain the team with his experience. Meanwhile, Xavi often replaces his position with other defensive midfielders like Kessie. Xavi understands that Busquets is no longer strong enough to play a full time, but he and his experience can help the team balance in midfield. But I prefer it if Xavi plays Gavi, Pedri and De Jong in the midfield because all three are really good at making Barcelona's attack power strong in a match. Over time Busquets will get weaker, so I think he will get a lot of time on the bench soon.