With Real's current squad, I only appreciate their midfield and attack. As for the defense, which is a patchwork due to injuries, I'm worried about how they will perform in the near future if this transfer period they decide not to bring any central defender contracts.
Real Madrid are quite confident with their options at the moment and it is unlikely they will undertake any transfer activity in the winter to add a defender. If later Real Madrid loses another defender due to injury, they still have several young players from the academy to overcome this. I understand it won't be strong enough but at least Ancelotti understands better what is needed on the pitch and if the coach wasn't Ancelotti it's possible that Real Madrid would have made an emergency transfer to overcome the lack of defenders.
Though Real Madrid defense is currently under having some bits of issues as a result of the injuries to few important members of the defensive players at the club, I don't think they'll be signing defenders during the winter transfer window because just like you said, Real Madrid have young players in the youth team to choose from in case of further injuries to the current players in the defensive position at the club. Again Real Madrid boss Carlo Ancelotti is known for his expertise in rotating his squad and perfectly making converting some players to play in their unnatural positions. I'll be surprised to see Real Madrid sign any player this winter transfer rather I'm expecting them to participate greatly in the summer transfer