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.
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.
Cristiano Ronaldo is one of the best player we have in our present football age and he have gigantic trophies to his name and gallery. He is big name and playing in the Europa league this summer doesn't gurantee him a single title because that's no match for the G.O.A.T. Cristiano Ronaldo is still active on the pitch and can deliver his team Manchester United. There's no time in his side as he would reduce his actions not to get injured, I think his time is near for retirement and this is probably his last world cup he will feature for his country Portugal.
I'm pretty sure it's in his last world cup to play against Portugal. He is 37 years old and will be 41 in 4 years. He will probably never see the world cup again. He will show all his strength with Portugal to become the champion, but his opponents are very strong. On the other hand, I am confident that Manchester United can rise to an important position in the European league. They can see semi-finals or finals and Ronaldo can be an influential name here.