Jose Mourinho switched his formation to a 4-3-3 that is very defensive when compared to his normal setup for multiple Clasicos two seasons ago, but he stuck with his normal lineup for both of last season's league Clasicos. For the most part, it worked. Bad misses in front of goal were the biggest reason why Real Madrid failed to beat Barcelona in the first Clasico of the season, and they were clearly the better side in their late-season victory.
Luka Modric has been brought in to the Madrid squad, presumably for what he can bring to a team in a match like the Clasico, but that doesn't mean that he's going to start. Real Madrid blog Managing Madrid doesn't see Mourinho adding a deep midfielder, and sees Modric as being in competition with two other players for one spot.
Mesut Özil, Kaká and Luka Modric are competing for just one spot. Real Madrid's number 10 is always one of Real Madrid's best players against Barcelona. Luka Modric is still a long way from being the reinforcement Mourinho was hoping for, but he's had his good moments as well. The Croatian playmaker should be chosen if Real Madrid wants to have a higher possession percentage, since Özil is a little bit best suited for playing with a counterattacking style. Kaká played some good minutes in Madrid's win at the Amsterdam Arena, but that's probably not enough to leave both Özil and Modric on the bench.
Mourinho could also opt to leave Angel di Maria on the bench and narrow things up, but it doesn't sound like he'll be reverting to the trivot.