I think the best option for Arda Güler is to loan him to another big club. Leverkusen?, no problem if they agree with the load option.
True, Arda Güler is still young and need high flying hours to improve his skill, but he have the potential as great player on future, real madrid must keep him on Santiago Bernabeu if still want regeneration club. The tournament is still a long way off, there will be many matches so maybe Ancelotti need the substitution for the attacker. And even if not often as substitution, Arda Güler should be agree to loan on another club in order to improve his skills.
Even loan is a risky move, but definitely not sell him. He may not be an awesome player at the moment, and Real Madrid is not like Barcelona where they play kids every game, but he does get playing time whenever someone is injured and he had two assists in the last game, so he is showing what he can do small by small whenever he gets the time for it. If he is loaned out, that means he is not going to play any minutes at Real Madrid level at all, he is going to play at a worse team, not like they are going to loan him to some great team, so Leverkusen or not, those teams are not at Real Madrid level and the results won't be awesome, and I believe that Arda getting few minutes at Real is a lot better than playing full time at Leverkusen.
Because even though it's few minutes and few games, he does play at Real Madrid, if he didn't play at all, if he was just benched, if he got zero minutes, then I would agree with a loan move, but he does get time so there is no need.