Team like Real Madrid should have good players or worldclass players in their bench as backup for such situation when injuries are coming to affect the starting eleven players, but they literary don't that enough quality players in their bench which is why some of their fans are already making that as an excuse in case if they lose their match against Barcelona.
If someone come and say that Madrid don't have squad depth, their fans will come for that person, and it is a plain truth they need to believe in; moreover, their defense is not as strong as they may expect which is why they are looking for possible solutions before the January transfer windows; but if they continue like this, they will definitely suffer at some points because some players will surely get injured.
Real Madrid has some players that are good on their bench but they are not used most often by Anceloti to demonstrate their effort on the big games. He is using a set and doesn't make changes often, though it's good because what he aim is always achieved and what is the used of changing players when clearly he can win the game with the main squad that he has, perhaps the bench ones will used this opportunity to show him their best.
I'm not sure you know Real Madrid defense for you to say they have weak defense system. As long a Mendy, Rodrigo and Alaba are still healthy and not injure like the rest or the defenders, Real Madrid are good and will fight in any match they have unless they are overwhelm by the team they play against. As long as the defenders will do their jobs and forward do the same, they will have chance to win.