The decline in Barcelona's performance cannot be separated from the fact that many of their main players are injured and this team does not have substitute players who have equal quality to maintain the team's performance so that it can become like now, this is quite unfortunate because this makes Barcelona even further behind.
I agree that the decrease of Barcelona performance is caused by too many injured players. Some of them are the key players like Lewandowski, Kounde, Pedri, Frank de Jong, and Raphinha. However, some of them have recovered now, Lewandowski and Raphinha have played yesterday. But they need time to be back to their best shape. It is not easy to play again with their best after they spend much time for the recovery.
I think, losing Dembele and Ansu Fati also contributes to the decrease of Barcelona performance. I still remember that these players often give the difference when they played in big matches in the previous season. I am a bit confused why Xavi doesn't need them anymore. Xavi may not know that losing them will bring a big impact.
I think this El Clasico is not ideal considering that many players in Xavi squad are injured, and those who have just recovered are not yet fully 100% fit. On the other hand, Xavi took quite a big risk when determining the starting XI yesterday, which turned out to be true because he had to admit the greatness of the strategy implemented by Ancelotti. Dembele and Ansu Fati were released because Xavi wanted several players like Felix and Cancelo and to make room for young players like Yamal.
Regardless from this defeat, Barcelona also still has a chance to retain the La Liga trophy, the difference in points is not too big and it is still reasonable to catch up in the following weeks. On the other hand, Madrid is in top form at the moment, so it's not easy to beat them, especially with a player like Bellingham.