I think Barcelona players are just tired because recently they played so many matches with too much energy. So I think Hansi flick needs to take rest for them, like make a smooth tactic where the player doesn't use his full of stamina. But anyway Barcelona are still at the top of the league with 33 points. Let's see what happens next.
Barcelona surrendered at home to Real Sociedad and although many people said that the referee's leadership in the match was a bit problematic, but in reality Barcelona did not have many opportunities to create goals.
Ball ownership above 70% makes Real Sociedad try to implement a defensive strategy so as not to concede and they get a moment that makes Barcelona goals goals.
Fatigue factors may occur and this is what is feared when Barcelona is too forced by core players without rotation.
This is not only a problem still at the top of the standings but one defeat can cause another defeat and the coach must pay attention to the level of players' weaknesses in the match so they do not repeat defeat in other matches.