Due to the circumstances surrounding both Real Madrid and Barcelona at the moment, there won't be any reason to say either of the two clubs would be favorite before going into this match, real Madrid would be without goal keeper Thibaut Courtois, Dani cavarjal and kylian Mbappé if he doesn't return before then whilst on the side of Barcelona, goalkeeper ter stegen, araujo, and a few of them but along the line some players may return from injury and give their team a boost. Nevertheless, in terms of performances at the moment I would rate both teams equally despite Barcelona leading real Madrid at the top of the table so for me, there is no underdog neither is there a favourite.
It would not be wrong to consider Real Madrid's squad as the strongest squad in La Liga. Real Madrid have a lot of talented players in their squad. But even then this team could not hold the top spot in the table. Real Madrid lost points in three matches. And the main reason for this erratic performance of Madrid is that several important players in the Madrid squad have suffered injuries. Real Madrid's performance has been somewhat erratic due to the absence of key players from the squad.
Hansi Flcik's Barcelona team is now in a very strong position. The bonding between the players is strong. Also the young players in the squad are in very good form now. This is why it is still very difficult for any team to beat Barcelona.
If Real Madrid play without Mbappe against Barcelona I would say it will be very difficult for Madrid to break down Barcelona's defence. Without Mbappe in Madrid's squad, I would consider Barcelona favorites in this match.