The best thing to do would be just focusing on what they can do with the players they have and just keep working on making what they have work. Surely barcelona has great players even without Olmo, they have been awesome all season long so far and I doubt that the reason why they are so good right now was Olmo, I am sure they will be fine without him too.
This is a very unfortunate loss for Barcelona, Olmo has already become an irreplaceable player, who managed to score three goals, and now due to a hamstring injury, he will have to miss from four to five weeks. During this time, Barcelona will play at least seven matches, these will be La Liga and Champions League matches, so Flick was definitely counting on him. But injuries are unfortunately an integral part of sports, for players, and I think that due to such a busy schedule, teams will more often encounter this phenomenon in their teams...
Although this season will certainly be a very long season for each player by having to focus on two camps, namely their national team and also their club, of course with this tight schedule will make each club experience a shortage of players but in my opinion Barcelona is lucky enough to have Flick he can overcome it all because every pressure under him will be optimal as he experienced in Munich in his past, so for me Flick will make all the players the same will not change much if they have to lose their players due to injury.