There's been a dichotomy of stating that the number 6 position, especially in Pep's team, is the most important and then expecting a player to be able to come in, compare to the best 6 of his generation, and hit the ground running when he's hardly played for months. If you want the back up to step in seamlessly, keep him match fit with minutes with the first XI.
When Gundo, Bernardo, Nico O, Kovacic have played at 6 for us, they had already been getting game time in other roles or regular sub appearances. That hasn't really happened with Nico G. Ah well, he'll be off and we'll look for another