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.
Barcelona did not start well in the champions league, that must be in the target for Hansi Flick, the reason Yamine Lamal and Raphinha are on the bench today replaced by Torre and Feran Torres. I do not doubt Barcelona's ability to maintain the perfect start they have had to the season even with the massive team rotation. Each player given an opportunity in this Barcelona team from what I have observed really understands the role that the coach expects them to play, and they do it really well. Robert Lewandoski still in striking for Barcelona, he should score.