There is no Szczęsny's name on the list of players, in fact Hansi Flick had to rotate several of his main players such as Lamine Yamal, Balde, Raphinha so that they remained fit after many previous matches. Basically, Hansi Flick doesn't underestimate Ossasuna, but using lots of young players is a risk he has to consider.
Szczęsny hasn't even joined Barcelona officially yet. I don't know why people were expecting to see him against Osasuna today.
By the way Barcelona must be in shock now. Osasuna have just scored the 2nd goal and it is 2-0 now. It has been only 30 minutes so far. Do you think they will come back and get at least one point?
Of course I wasn't expecting them to win all their games. Completing a season with such statistics is one of the toughest works for any team no matter how powerful they are. But at least I would expect some reaction from them. They don't play well for now.