teams dont carry the 4th choice in one position for depth.
So you are considering Bernal ahead of Casado? I don't think you should, though. Looking at Gavi and how much time it took for him to be 100% and he got injured again because of the same injury, I think Bernal should be considered behind Casado. He should be integrated very slowly.
Yes, come next season, if Bernal is fine and dandy, he should be ahead of Casado. And we can even think of selling Casado.
Also with Gavi. As you mentioned later, I don't think we should consider him as the DM backup. Everything he has shown as of yet, he is a CM. Let's wait till he plays some matches and then formulate if he is an adequate backup as a DM.
So for that, I would say Casado should be the main Backup for FDJ, while Gavi should be for Pedri.