I think midfielder is the key for team playing what have to do when attacking and defensive moment, if midfielder can't manage the game style will be broken and hard for forward how to scoring goal on every match.
The midfield line has a big role but we don't deny a decrease in the quality Barcelona defenders and attackers. Even Barcelona midfielders have done their best in each match, it will end with a defeat if the defenders and attackers perform badly. So, we cannot blame the midfielders only, attacking and defense line are also the causes of Barcelona's current slump in performance. If Barcelona wants to improve their performance again, they must rebuild all the lines (attacking, midfield, defense). They can't rely on the midfielders only to make the difference in a match.