Real can win both 4-0 and 2-1. Any result is possible, even without taking into account whether the main team or the team with some reserve players will play. If this game was not after the international break, then I would bet on the victory of Real Madrid with a handicap of -1, but most likely -2, but since this is not such a game, it may be generally worth waiting for the game to start and make bet in live mode.
I see no way to call Osasuna weak. Osasuna's performance this season has been fairly stable. They are now the fifth team in the table after winning 5 out of 6 matches. And so it cannot be said that Real Madrid will win easily. However, they have not lost a point in any match so far. However, teams often perform erratically after a long break. And so I wouldn't be surprised if Real Madrid dropped points for the first time in the season.
This is Real Madrid's home match. Real Madrid is strong at the Bernabeu. Madrid is also unbeaten in their last 15 matches against Osasuna. However, Madrid drew 0-0 in the last home match. So Osasuna will play the next match with the goal of a draw.
Osasuna is a good team, but we are talking about weakness in relation to Real Madrid, right? The fact that they are in fifth place does not mean that they are "close" to Real Madrid in terms of strength. No matter how badly Real Madrid's opponents play, there will always be someone in second place. Even if Real Madrid is a million times stronger than everyone else, someone will be in second place, third, and so on. I am sure that if this game was in the Champions League final and it was the last game of the championship (there would be no need to save energy), then Real would have won 10 games out of 10 and the goal difference would have been consistently 3+.
IMO, Osasuna is not bad, but not a good team neither. The fact that they are in the fifth place right now is more accidental than due to technical of physical competitiveness. We are still at the start of La Liga, so luck plays an important role in the ranking. Let's see if they manage to stay in the top 10 when we approach the end of the season. Their great start will help, but it is not a sure thing.