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.
I wish I knew what the odds were for Osasuna to win with a handicap of -2. Probably something around 50.0

As for rotation, Flick changed most of the defense/midfield too. I wonder if he really believed that it would be an easy game and he could experiment here without problems?
Update: Yamal scored a goal, but the score still didn’t become decent after that, 4-2 is too much.