Real Madrid have Eder Militao and David Alaba as centre-backs now and they are the starting eleven players actually. But Ancelotti didn't prefer Alaba in the starting eleven yet. Nacho Fernandez filled his place.
These centre backs aren't enough to build Real Madrid defence. Nacho and Militao aren't strong defenders, it is clearly seen in the previous match against Levante. They were conceding 3 goals and almost be defeated by Levante in that match. If Ancelloti still dreams of the La Liga or UCL trophy, Real Madrid must plan to buy top centre backs. Then, Nacho and Militao are better to put as substitute defenders again.
Forget about transferring good defenders to Real Madrid until Perez buys Mbappe, Holland or another star. Real Madrid needs to strengthen not only the defense, but already think about buying players in midfield. Cross and Modric are getting old and unable to play against fast and physically strong players. But Peres needs a star, or better two.