We can see how far they can play well without their main players in this match. I agree with what you said, Barcelona's game can be judged by this match before the next game which will put more pressure on them. They don't have much time to prepare who will be starting in the lineup. Xavi should already know who those players are, because most of the players he has retained this season have been his picks.
Temporary statistics on paper Barcelona still wins from Mallorca. but that's all we have to keep an eye on, unstable conditions can occur in the field. Mallorca should be sure to lose. However, Xavi can still overcome some of Mallorca front-line players. Lewandowski was lucky not to be absent because in that way Xavi was still able to focus his attack with the center on Lewandowski as the executor. There are still Dembele and Ansupati who can provide a little boost up front.
This match will not be too heavy for Barcelona even though some of the main players are reported to be injured, Barcelona still has several players to be the main starter against Mallorca.
Lewandowski will remain the first choice as Target Man, and he will score in that match.
IMO, it won't be difficult for Barcelona to beat Mallorca, however, the tough game for Barcelona is in the champions league. they have an away game against Inter Milan.
of course Xavi will be at a disadvantage because some of his core players have to rest, and this is clearly not a good situation for them.