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.
You are right, Xavi only accepts the rest of the decisions of Koeman and the club management. So at the moment he is just maximizing the players. Xavi tries to maximize the potential of existing players, because he didn't come at the start of the season.
Blaming Xavi is certainly not entirely right, but if you look at Xavi's experience, he still has little experience when he became the coach of a club as big as Barcelona. But it's interesting what Barcelona will do in the winter transfer market. They should buy new players, to improve the team's weaknesses.