That's quite some confidence given that is only two points ahead, they are playing away from home and Atletico Madrid has the best defense in the league. Additionally, Mbappe is injured. That game is going to be very hard for Real Madrid as Atletico is kind of the underdog and has less to lose in this game. The winner of the matchday could clearly be Barcelona if either Atletico wins or they end the game with a draw.
But Osasuna not a walkover for Barcelona. 7th rank and although I doubt that Barcelona lose points here, it is of course always possible. But a win would be huge. It would be nine and seven points respectively against Atletico and Real and both know that a loss would be very painful then.
There is so much for Real Madrid to lose this match, they just have to win it even though it's assure they have the body to make this 3 points or at worst, they can do draw because Atletico hasn't lose any match this season, they have draw and considering they will be hosting Real Madrid, they will have some bumps set ahead for Real Madrid and even in the past, their last few games didn't sit well for Real Madrid, their striker might also not be available due to a minor injury.
I think the faith of Osasuna is the same thing with the faith of Atletico Madrid. They are hosting the two big guys right in their home and they will be playing against them and these two need three points to max out in the top of the table. They have also not lose any match this season and so is Barcelona and Real Madrid, it's like we are having top matches this weekend and all of them are looking for the same thing, 3 points and nothing else.