I can agree with you about this. Arda Guler is still 18 years old, it is too early claiming he will experience the same thing as Eden Hazard. Moreover, Arda Guler doesn't experience a too long injury as Eden Hazard. As far as I know, he already recovered from the injury. But he seems still waiting for the best time to play again. I assume Ancelotti doesn't want to take a big risk by suddenly put him on the lineup. Ancelotti lets him to sit on the bench first, Arda needs to ensure first his fitness. However, I think we will see him to play again in the near future. Maybe he will play in matchday 14, when Real Madrid against Cadiz.
I'm looking forward to it against Rayo. Because he was included in the list of players against Rayo. But it turns out he didn't play. I was also looking forward to it against Braga in the UCL but there was no debut either. Anceloti chose to play young player Fran Garcia. It's true that Anceloti probably doesn't want to make many big-risk decisions. It seems that Anceloti wants Guller to be 100% ready to play. This may also be due to his experience with Eden Hazard. I think Anceloti's decision was right, because Guller is a future asset for Real Madrid.