The 37-year-old has allegedly informed the Red Devils of his desire to leave the club in the wake of their sixth-placed finish in the 2021-22 Premier League campaign.
Ronaldo is understood to have concerns over the club's ability to compete for silverware in the upcoming season and has been left frustrated with a lack of ambition in the transfer market.
The five-time Ballon d'Or winner reportedly wants to win at least one more Champions League crown before he calls time on his playing career and is yet to report back to Carrington for pre-season training citing family reasons.
Man United are adamant that Ronaldo is not for sale this summer, and the 20-time English champions have apparently informed the attacker that he will be required to attend their pre-season tour.
However, Sky Sports News now reports that Man United are not sure if Ronaldo will travel for their friendly matches, as the attacker continues to train in his homeland at Portugal's training base.
Erik ten Hag's side kick off their pre-season in Bangkok with a clash against bitter rivals Liverpool on July 12 before facing three fixtures in Australia with Melbourne Victory, Crystal Palace and Aston Villa.
Atletico Madrid will then be awaiting Man United in Norway on July 30, and Ten Hag will take charge of his first non-competitive game at Old Trafford against Rayo Vallecano a day later.
The report claims that Ronaldo's agent Jorge Mendes is currently formulating a package to present to potential suitors in the hopes of pushing through a transfer for the Portugal international in the coming days.
Man United are thought to be reluctant to sell to a direct Premier League rival, but the Red Devils could be more open to offloading Ronaldo for the right price to a foreign side.
Ronaldo has just 12 months left to run on his current contract with the Red Devils, although there is the option to extend the ex-Real Madrid and Juventus man's stay until the summer of 2024.
During his second spell at the club, Ronaldo has amassed 24 goals and three assists from 38 appearances, including 18 strikes in the 2021-22 Premier League season.
A return to Sporting Lisbon has also been mooted for the Portuguese, but his extortionate wages - thought to be in the region of £480,000 per week - would likely scupper a move to Ruben Amorim's side.