I know Fenerbahçe's coach, Mourinho, is a world-class coach. But it doesn't mean it's right destination for Arda Güler to go there. Unless he wants a career suicide, he should stay here, in Real Madrid, as the next upcoming talent. He needs to be developed in a competitive league to form his performance and mentality.
I admit that benching Arda Güler at Real might hinder his development. But there are better loan options for him soon. Anyway, I think he should stay, and i belive he will shine later once he gets a proper club to be loaned. He still has enough time to develop himself. There's no need to rush.
Mourinho has always been very ambitious, especially in his best years he demanded only the best players from the management, but everything began to change when team after team that he coached could no longer achieve great success. If you remember Roma, there he had to work with the players that were in the team, maybe they managed to attract a few free agents, if I'm not mistaken, that's how Dybala came, or a loan like Lukaku. So I doubt that he is now able to get the players he wants.