If a team wants to look down on their performance just because they don’t have their first eleven complete and healthy to play, that fear will kill their chances of making it in that match and will lead them to a very big defeat in the game. Real Madrid is not a team that should rely on only a few set of players, their back up substitution in every game should make them a worthy team in any match they play in. Barcelona might be ahead of them in the standings but that doesn’t mean they can’t overtake them this soon in the league and take on the first position even though their performance hasn’t been that welcoming as that of Barcelona since the start of the season. Both teams are definitely going to take this match seriously and I expect them both to have equal chances of winning in this game.
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.
Well, Some people might say that Real Madrid does have some young talents in tone of benches but these players will not be as effective as the first team. It also brings lots of pressure that force the club to look for possible short terms solutions, particularly in high profile matches such as El Clásico against Barcelona. A defeat such as this would most definitely put pressure in the team and maybe make the management push for players in the upcoming January transfer window in case the require them.
Concerning defense it is actually one of the most vital areas to develop. Which is even more valuable, bearing in mind players that got injured such as Eder Militão who play major role as a defender. Otherwise that can turn into some sort of weak spot which opponents will be able to take advantage of as soon as they meet without proper adjustments in strategies or players’ signed contracts.