Indeed. We have no reason to claim whether he is suitable for Real Madrid or not, he still didn't play in any match for Real Madrid. Arda needs to prove his quality in some matches for Real Madrid. Although he is a very skillful player, it doesn't guarantee he will be suitable to play with Real Madrid main squad. But we also has no reason to claim he will fail as long as he didn't show it in any match.
I think Real Madrid needs to prepare first before actually becoming a main player We don't underestimate his abilities but his experience is still not enough competition in local leagues is certainly different from in La Liga so he needs further adaptation, as a substitute may be the right way to provide experience and adaptation. When the time comes he will be in the main squad as a starter, we certainly still remember how Messi started his career at Barcelona and this player may also have gone through the same process.