Ronaldo would definitely be a good addition to Manchester United's attack, he is in great shape and regularly scores for his team and for Portugal, but it is obvious that due to his age they will not be able to offer him a long-term contract, which of course will not suit him, so I think his return to Manchester United is excluded. I have also not heard that he is interested in a coaching career, so I also exclude this option. Amorim can poach a few players from Sporting, but it will still not be enough for him to fix the team, for this they need transfers, and I think Amorim will get them in the winter transfer window.
Manchester United tried with Cristiano Ronaldo four seasons ago and they failed with this bet. Reasons can be from Cristiano Ronaldo or Ten Hag or both but four years later, Cristiano Ronaldo has been already four ages older so it's impossible for Manchester United to bet on him again.
Furthermore, two years ago, before Cristiano Ronaldo left the club, he made an interview like a bomb and it revealed many bad things in the club, not only Ten Hag but the owners and other people, so the club owner will not want to work with Cristiano Ronaldo again. They hired a player, paid him, and would like to see respect and professionalism from the player, not disrespectful interview that was done by Ronaldo 2 years ago.
It's certainly that Amorim will try to bring some new players to the club but none of them will be Cristiano Ronaldo.