Why would Arda Guler be a member of the starting lineup right away? I don't think he will be there right after he recovers. He will need to earn his place in the starting lineup because he has really skilled teammates in the positions he can play. Ancelotti is thinking of him for the right wing now.
But players like Rodrygo and Valverde is also used in the same position. Arda's injury is really unfortunate now. If he was able to start with pre-season games then he would have had more time before the season starts officially to prove his quality to his manager.
I think it is not "start all games" situation but more like Real has about 50-60 games a year, so he will get some minutes type of situation. He could still crack that famous 2000 minutes mark, which would not be impossible considering he would have played not only at right wing, which he could play, but also he could play at midfield as well, right behind the striker type of deal, or even at the midfield itself.
When a player has that many positions he could play at, that makes him valuable as a bench player, so whenever there is someone that is injured, or whenever there is someone that needs a rest, or if it's an easy cup game etc, all of those could go to him. However, considering he is only 18 years old, his injuries while he is growing up and training his body, could make him not play much even after this one is over, he may get it again, they need to load him up first to make him better.