Barca is in a rotation of players in their defense. they had been strong at the start of the season. but to play consistently, of course, it is impossible for players in the Barca main squad with so many matches.
Barca now has a weakness, and Xavi will of course have to take this into account for the winter transfer window. looking for a formidable defender to become a strong fortress of Barca.
The mainstay players from Barcelona are not able to play consistently because it requires very high energy to guard at the back which is very vulnerable to opponent attacks, Many defenders are injured because of the previous game, therefore Xavi must carefully consider the formation for each game so that get good results.
I would say the performance of the Barcelona team is very good. But when they face renowned and strong teams, we don't see consistent performances from them. The main reason for this is their weak midfield. The performance of the Barcelona team would have been better if the fielders' performance had been stronger and more consistent. Barcelona also has some tactical weaknesses.