Ronaldo is no longer the best player in the current Manchester United squad, he doesn't have much evidence that he is still the best among the other players in the team. Ronaldo missed many opportunities to play in Manchester United matches, even Ronaldo did not get a chance when Manchester United lost in the derby against Manchester City.
Nobody can win ageing forever especially in football and at highest levels like in Premier League or Champions League. From minutes he played so far in this season, I see he loses his best capability but I believe it comes from not only his ageing issue but also from his lack of training in pre-season summer and his stressful mind.
I checked Ronaldo's stats, he only scored 1 goal in 7 Premier League match. So I'm absolutely sure to say he's not the best at Manchester United at the moment. Ronaldo's performance has indeed declined, but as the player who has played the most for his team's wins in the past, Ronaldo still holds the title even though he has now been replaced.
I believe he will come back and he can play better than what he has shown so far. If United can build up a system that can provide enough good key passes to Ronaldo, he will help them to have many goals. However, Ten Hag wants more contribution for a whole system including proactively defending. Therefore, Ronaldo is almost not in plan of Ten Hag.
I think the coach more likely intends to use Ronaldo as alternative player when the club needs to make changes and find goals. It's challenging for Ronaldo to find a position in Ten Hag tactic.