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There is no way Ronaldo will consider returning to Manchester United after his previous experience that led to him moving to Saudi Arabia. I also think he will rather accept a move to Sporting than return to United, and with the way things are playing out I think he might even retire at Al Nassr.
I'm not sure who still expects Ronaldo to return to Manchester United for the rest of his career now, if he does then I'm sure he won't be an active player anymore. Ronaldo will only be closer to retirement and he will be a legend for several clubs including Manchester United, but expecting him to return now is unreasonable and has no better chance.
It is possible for Ronaldo to return as one of the staff, but not as a player.
He is more suited to being an artist like David Beckham, not any football club staff. Even if Ronaldo is interested in becoming a team coach, I think he will miss it for the next 10 or 15 years.