Who is ahead of Asensio in the pecking order? Arthur has to pay his dues and has Rakitic ahead of him. Let's not make something out of this when there is nothing there. He will be fine.
A team cannot win the CL without 14 starters at least. We should have the first eleven + Arthur, Vidal, Malcom and Lenglet. Maybe Semedo too if he steps up. These players need, until february-march, to have many games checked as starters or else we'll complain that they aren't stepping up when needed.
First we need to integrate them properly, play them, give them full games, and only then we can expect them to get the job done when needed.
So far, all three have barely played. No time to lose. CL knockout rounds will come soon.
Arthur for example needs to be brought up to a level that is good enough for a CL quarter or semifinal. Injuries and suspension will happen. Even if you consider Rakitic as a sure starter, if something happens with him, then Arthur will need to start. And if you haven't made sure he is very well in rythm and accustomed to top games, then you'll suffer a massive drop in quality.
Madrid played vs PSG away in the last 16 with Kovacic, Asensio, and Vazquez. Neither Kroos or Modric played, their two starters in CM. We need to be able to do the same at any point almost. We should be able to play Arthur + Vidal in such a game and suffer a very small loss in quality if at all. That's on Valverde to make sure it happens.
Vidal and Arthur are probably better players than Kovacic and Vazquez, so it is just a matter of skill from the manager in charge to get his team management decisions spot on.