It's not good to play at Pamplona in January. And when it's supposed to rain and the temperatures are expected to be at around 5ºC, a trip to Osasuna's field without Cristiano Ronaldo, Sergio Ramos and Pepe can be a nightmare.
Osasuna will also be missing Loue, Riesgo, Sisi and Torres, but none of them would be a big threat for Real Madrid. As a matter of fact, it's hard to point out a single player that could cause Real Madrid some trouble other than Fernández. Osasuna has conceded 17 goals and scored 13 (Real Madrid has conceded 20), so they defend quite well. Thing is, they can't score.
When Kike Sola is your highest scorer with four goals, you know you have a problem. Mendílibar has tried to build this team around the goalkeeper and some good defensive performances, and that should be considered a right decision if a look to Osasuna's attackers is taken. Armenteros, Nino, Cejudo and Sola aren't probably what you might consider Liga BBVA level.
Armenteros is the best one out of those four, and he's only scored two goals and delivered two assists. He helps his teammates defensively a lot, but that's not the only thing you expect from your best player.
In the midfield, Oier and Puñal will try to stop Real Madrid's playmakers. They don't have much quality, but they certainly don't need that to bother the opposition's flow and rhythm.
As stated before, Osasuna's defensive line is clearly the best part of the team. Bertrán, Arribas, Flaño and Damià are playing great to allow their team collect five draws in this first half of the Liga BBVA season. They are physically strong, but Real's ability to switch position could make them suffer.
One way or another, Osasuna's most dangerous ally will be the Reyno de Navarra, the cold temperatures, and Cristiano Ronaldo's absence.