In my opinion, there are two factors that are working against Ronaldo right now.
One of them is his age as the guy’s 37 right now and his body can only do so much for himself at this top level of football. As a matter of fact, the Premier League is an extremely physical football league right now, so playing there at the age of 37 and expecting the best out of him is a no-go.
Ronaldo has excellent fitness and stamina even though he is 37 years old, I will not doubt he can still play full time if he is in good form. it's just that, his performance has decreased which makes him not the main starter at this time.
my assumption, ronaldo has no match with erik ten hag's strategy. we can see how early in the season he played against Brentford, then against FC Sheriff.
performance is clearly not looking in good condition.
Another factor could be the fact that he would like to play in the Champions League but is unable to do so. As a result of the fact that he wanted to leave United because he wanted to play in the Champions League and now he is not getting the opportunity to play, his morale has dipped and is affecting his performance.
Well, I agree with this. His desire to leave Old Trafford, made the relationship between Manchester United and Ronaldo worse. and in the end ronaldo did not get the trust of erik, in the end he had to be willing to become a second tier player.