I would say it was a wrong decision by Hansi Flick to leave Yamal and Raphinha out of the squad for this match. And this is why Barcelona are going to lose. Osasuna team cannot be called weak as we have seen good performances from this team in previous matches this season. But Hansi Flick considers this group to be weak. And that's why he didn't keep Yamal and Raphinha in the starting XI.
In his reasons Hansi Flick did not underestimate Ossasuna, but it was a difficult decision for him in this match. Lamine Yamal and Raphinha needed rest to be physically prepared for the Champions League, but as a consequence Barcelona's performance was very bad. Barcelona's defense in this match was really problematic and Hansi Flick didn't have many other options when several defenders were injured, so this defeat had to be accepted.
Ossasuna will eventually secure 3 home points in this match considering they have scored 4 goals. Barcelona are 4-1 down to Ossasuna and this is really a result that was very unexpected to happen. What can be done, it seems like everything is over.