The Portugal international is understood to have informed the Red Devils about his desire to be released from his contract.
This, according to Mail Online, would allow the forward to find a Champions League club ahead of the new season.
Despite his wishes, United have not changed their stance and are not ready to part with the forward just yet.
Manager Erik ten Hag has continuously told the media during pre-season that Ronaldo is simply not for sale.
However, with the prospect of Europa League football looming, the Portuguese forward appears to be looking for a way out.
One club which would not offer Ronaldo a warm welcome are Atletico Madrid, after fans unveiled a banner reading "CR7 NOT WELCOME" during a recent pre-season match.
The 37-year-old spent nine seasons at Atletico's arch rivals Real Madrid, where he scored 450 goals in 438 appearances.
During his time in the Spanish capital, the player helped Real to 15 trophies, including four Champions League titles.
What's more, throughout his career, Ronaldo netted 25 goals against Atletico Madrid. The forward has scored more goals against one other club, that being Sevilla with 27.