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.
Barcelona is experiencing problems in defense due to injuries to several of its main players. But in the last game against Villarreal, I think their defense has started well even though there are actually some weaknesses. Bercelona often concede on the counter-attack by the opposing team because the current defenders don't have good pace, especially at centre-back. This is a problem that actually makes their defense weak, but the most impressive thing in the last match is Barcelona managed to keep a record of not conceding at home in 5 matches so far and Barcelona have scored 11 goals at home.
In the next 2 days Barcelona will host Athletic Club Bilbao at Camp Nou, this is their 6th home match in Laliga and I think Barcelona's chances of winning should not be affected. Bilbao have not been in their best form in the last 3 matches, but they are a strong team in defense which will likely make it difficult for Barcelona to score goals.
Barcelona's defense was good throughout the early season of La Liga. But once their match with Real Marid and lost 3-1 to it, I think the confidence from the Barcelona player's viewpoint about their defense suffered as well. Maybe there was some doubt about it and we'll see how Xavi tries to stock up the player's confidence. I'm sure Barcelona will have an easy match against other teams in La Liga. Tho in their next game where they have to meet Bayern, that's where Barcelona's defense will be truly tested again and Xavi has to come up with something new here.