Real Madrid has not lost a single game of the last 17 games they played. They have the best defense in La Liga and they are the only team with Manchester City to finish the Champions League group with the full 18 points out of 6 games. I understand that they are suffering from injuries, but this isn't the first time Vinicius Junior is injured this season. They had to replace him for quite a while already and yet were able to play successfully.
There is still some time left for their players to recover and I don't see them being on the verge of instability. They have done it well until now and of course there is a limit for every club to handle injuries. But for now they are good.
Yes, even with the lack of defenders that Real Madrid currently has, they still have a good defense. I still don't believe they have a stable performance, maybe if this happened to another team then it would definitely be easy to concede the goal because of the weak defense. But Real Madrid also has two goalkeepers who have played very well this season, namely Kepa and Lunin, both of whom often rotate according to the team's needs and rarely concede goals.
Several players have been confirmed to return, except for those with ACL injuries, such as Militao, Alaba and Courtois, who will probably be back for quite some time to strengthen the main squad. But that doesn't seem to be a problem because Ancelotti knows how to manage the existing squad so that he can still get maximum results in every match.