Xavi cannot be blamed for Barcelona's defeat. Barcelona's squad is very weak and inexperienced compared to other title-seeking squads. We can't expect much from such a team. Xavi has taken charge of the team. He will now stabilize the team somewhat. Fix their gameplay and strategy. Then next season if Barcelona can take some star players in the team then maybe they can be strong. I don't expect much from this team until new players are added.
Agree with you. Barcelona lost the first match in the same way against Bayern when Koeman was the coach. Barcelona is a broken team now, only coach can't fix the team lineup and performance. They need some good players in their squad. They are also having bad journey in LaLiga. Maybe Barcelona won't be able to qualify for next UCL season. They should bring some good players during the upcoming transfer window.
A coach can not just magically transform a team that was performing pretty bad before into a top team within a few weeks, as you already said. I am pretty confident though that Xavi gets a lot of time in Barcelona to realize and to improve the way that he wants his team to play football. Xavi has such a high standing in Barcelona though that i i think he can really take his time to change things as he wants, even if he is not successful with it in this season i doubt that Barcelona would fire Xavi so quickly.
I doubt though that Barca can make some good transfers because they have 1,35 Billion € of debts, so there is no way that they can just buy a few good players, they have to rely on their own youth players.