If the current substitute players cannot fulfill their expectations, the defense line will improve. So there is no harm in buying quality defenders in the mid-season transfer market. Because we know Eder Militao and Daniel Carvajal will actually be absent until the end of this season. And of course Real Madrid must look for players who are equal or close to the players who are absent due to injury. The defensive crisis experienced by Real Madrid this time is truly the worst in recent seasons. So it is not surprising that they will start to have difficulty winning matches if their back line weakens.
Not all substitutes can meet the coach's expectations and maybe this happened to Real Madrid so that their performance so far has been inconsistent. The absence of Eder Militao and Daniel Carvajal will leave a big void in that line and maybe even if there are replacements they are not as good as them. This will be Ancelotti's big task to maximize the existing potential because if the upcoming transfer market is not used to recruit some quality players in that position.
Real Madrid also has a target to defend the La Liga title and maybe the Champions League is also one of them that they need to think about more seriously. With so many injured players it will be a little more difficult for Ancelotti to make the substitutions he wants.