Perhaps Ancelotti will give some players a rest, but I don’t think that Real will play today mainly with players from the bench, because the El Classico for La Liga is also a very important event that Ancelotti cannot neglect. In addition, if you give this game to Barcelona, they will be 5 points closer to Real, which is not critical, but why allow this if you can beat Barcelona today and get closer to the championship.
Because this is an El Classico match, there is no reason not to field the main squad. Ancelotti realizes that the match is not an ordinary match and will continue to involve their respective statuses. Likewise, Barcelona has no other targets in European competition, so this is the only match they have to maximize. Plus, if Barcelona is able to get 3 points then they are very open to continuing to chase Real Madrid. There is nothing impossible as long as they have matches remaining, Barcelona is definitely waiting for Madrid to slip up and lose one match and today is the opportunity. With 7 matches remaining, many unexpected situations can still occur, Madrid cannot feel comfortable at the top of the standings just because they are leading on points.
It's true what you say about this match, for both teams this match is a very prestigious match, so there is no reason for them not to appear with their full squad because it will be detrimental to them. Even if the result of this match has no influence on them, still when two rival teams meet they will want the best result, especially as we know this match is a hot match in Spain.
Now Real Madrid has the upper hand, they are better than Barcelona and they will also appear as the home team. Ancelotti really knows the abilities of the young players he has, so he can take advantage of that. Moreover, in the last few matches they have also recorded better results, especially when they destroyed Barcelona in the last match they played in the Spanish Supercopa final. That is their capital to be able to beat Xavi's men again, even though it guarantees victory for them.