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.
From several matches it appears that the current madrid defense looks quite fragile, even though real madrid has Eder Militao and David Alaba, but in my opinion these two players are not too strong enough to strengthen the real madrid defense at this time, especially in the last game of madrid, alaba made some mistakes and one of them ended up being a goal in the match against Levante, in my opinion, in addition to chasing mbappe, madrid should also try to find a typical defender like ramos so that their defense can become stronger this season, if madrid manages to get mbappe and make a line their front is getting better but without the support of a strong defense, of course it is a futile thing.
In a few days, Real Madrid will play against Real Betis. As guests, of course, they will get an onslaught from the host. If in this match Real Madrid will still concede (especially if many goals are created) then the Real Madrid defense line does need serious attention. The transfer window will also close in a few days, so Real Madrid has very little time if they want to make improvements. Hopefully, there is good news before the transfer window actually closes.