This season it really doesn't matter whatsoever how well he does in Liga or CL. What really matters is whether he makes key decisions about the future of this team moving forward.
He needs to first establish that the Amigos are out the door. Sell who you can and for the ones we can't, they become veteran bench cheerleaders.
Then, give at least a skeleton idea of team structure that Barca should establish going forward. FDJ becomes the spine of the midfield and Busquets is gone. Also establish the precedent of trusting Ansu and Puig and the roles that they will hold in the team. Even potentially show Barca how a good double pivot can look like.
This idea that "double pivot is slow for 2 big statues" is outdated as hell. Liverpool, Bayern under Flick, France, 18/19 Ajax, Holland all use a mobile double pivot sort of setup. FDJ with Van de Beek being a high energy Box-to-Box support player next to him can easily mirror this modern meta. Puig ahead of them will also be a relentless dog with his pressing and all 3 players in the midfield are good for 10-11km per game at least. A fluid 4-2-3-1 is THE current meta and Barca have been fools to be so out of the loop on tactical trends.
If Koeman can establish these precedents then I don't care how sloppy we look with the growing pains of a new team. Future coaches whether it be Xavi or Pep or whoever need to know how to use FDJ correctly and if Koeman can trigger a breakout season for FDJ it will become undeniable to see the role he is made for. He is not the next Iniesta and never will be.
Once we truly give him the keys to the midfield the team will start looking way different from top to bottom.