I agree with you, especially in the case of Dembele and Ansu Fati. Barcelona would have performed better with those guys if they were around. even though I won't easily discredit the efforts of the players that played yesterday match. But Barcelona should have envisaged the big match and could have add more efforts.
Although there is no guarantee that Dembele and Ansu Fati can help much for Barcelona, but they possibly make Barcelona front line stronger because they played quite productive in the last season (especially for Dembele). It is very different with Ferran Torres, this man can't contribute well for Barcelona. he often played as starter currently because Barcelona have no much options for the attackers. I'm sure Ferran Torres will be always on the bench if Barcelona still has Dembele or Ansu Fati.