Xavi assessed that Barcelona's defense last season was really problematic because it continued to concede more than 35 goals in the last 4 seasons. That's why his attention on defense has increased this season with Kounde, Crhistensen and Kessie his signing targets. Then Xavi is still not satisfied where Marcos Alonso is the next target to be brought in even though the financial problems they are currently facing have not been resolved.
Barcelona's back line was quite fragile last season, they didn't have players capable of breaking attacks from opposing clubs, so they conceded more than 35 goals, but it seems that wasn't Barcelona's only weakness last season, since Messi's departure. Barcelona seems to have lost their best game, even though the composition of the players was not that far before Messi's departure, Messi's dependence on Barcelona was quite visible after his departure, Xavi had a tough task, rebuilding this club to get back on track