Real Madrid has been unlucky with injuries this season, three players were hurt in the previous game against Osasuna, however, Real Madrid won the game, which was important because it was the first time they had beaten their opponents with a score of four goals in La Liga this season. But the worse news was that most of their players would be out for a few weeks, and Militao would likely be absent for the season, forcing Valverde to switch to right back in the last match. I believe Real Madrid is struggling with a situation similar to what Barcelona experienced last season when they had a lot of injured players. But I believe Real Madrid has the funds to sign additional defenders, compared to Barcelona, who is struggling financially.
However, Ancelotti has to be held responsible for all issues that Real Madrid is currently having since, as a manager, you do not have to believe in your young players, if he had used some of his young players, Real Madrid would not have as many injured players as it is. Raúl Asencio, a 21 years old center back, just made his Real Madrid debut against Osasuna and provided assist. In my opinion, he is more impressive than Militao.
Maybe you are right, and in another situation Ancelotti could have trusted the young players more, but this season for Real started badly and now they have to take every match very seriously, because in La Liga Barcelona was far ahead and in the Champions League their affairs are not going well either. In such a situation Ancelotti could not afford to trust the young players, maybe it led to so many injuries, and maybe at the end of the season it will turn out to be the only right decision that Ancelotti made.