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.
It's difficult to tell as you can see at the example of Bellingham. He was so young and yet so grown up that hardly anyone could believe what they are seeing when he played. It was amazing from such an early age on for Bellingham. Not many players can do what he is doing at that age. He is already one of the most important core players of the team, if not already the most important player.
But I would not expect the same from Arda. He should get his chances to prove his value to the team, but expectations shouldn't be derived from what Bellingham is doing. The difference is also that Bellingham had time to develop at that age when he played for Dortmund. Arda would skip that phase.