#2 The defensive midfield problem
This is a flaw that clearly amplifies how poor Barcelona and Real Madrid have been at squad planning. Given the manner in which they dominate games, La Liga's giants don't ordinarily have much of a need for a defensive midfielder. However, the aura around these sides is disappearing and even minnows aren't afraid of taking the game to both Madrid and Barcelona
For Real Madrid, the problem is very clear. Zinedine Zidane has a 100% record when Casemiro starts for Los Blancos. In his absence, Madrid's midfield falls apart, as they're not as defensively secure, as evidenced by the manner in which they conceded 6 goals in their last four draws. Madrid erred in not signing a backup for Casemiro, as Toni Kroos and Mateo Kovacic aren't similar players, resulting in Zidane having to rejig his system into a less effective one.
Barcelona, on the other hand, deserve less criticism for not anticipating their defensive midfield issues. Sergio Busquets has been Mr. Dependable for Barcelona for so many years, and to see him struggle for form is truly surprising. Yet, there's no plan B for Barcelona, as moving Mascherano back into midfield will also reduce their attacking impetus. Luis Enrique's only chance is to patiently hope that Busquets gets back to his best sooner rather than later