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.
To be simple and clear I can say Barcelona is clearly not strong enough to play in the Champions League and they won't be able to beat the stronger team considering the situation in this tournament and we all know why, That's because Barcelona doesn't have superstar players and experienced players in this team and Flick had to use the young talents to have a cheaper team and avoid spending too much money and the bad financial situation of Barcelona is the main reason for it.
They have Robert Lewandoski who can have a good performance, but Barcelona has no other players to fill the place of Lewandoski in this team and there is the same situation for Lamal.
But the performance of FC Barcelona doesn't show that they can not perform well in the champions League this season following how they started this season, I understand that they lost in the champions League competition against AS Monaco but that look came as a result of the red card they received in the game. The team has no big names to Ike they used to have in the past but the coach has been trying to manage the players in his team right now and I believe they can end up getting better and better if they are able to manage the players in their best performance.
About the players you mentioned, the team is struggling to survive the financial crisis they are into finding a backup for Lamal and Lewandoski is not possible at the moment unless they will recruit young players from their youth team.