Ronaldo, who won three league titles and the Champions League during his first United stint, has since been linked with moves away from the club.
Acoording to The Times, the 37-year-old has now asked United to be allowed to leave if the club receives "a satisfactory offer", which will be a huge blow for new boss Erik ten Hag, who was looking forward to building a team around the Portuguese.
The report suggests Ronaldo wants to spend the final years of his career playing in the UEFA Champions League, with United having to settle for UEFA Europa League football in 2022-2023 after a disappointing campaign.
Ronaldo returned to Old Trafford in 2021, with visions of more title glory at the Red Devils having enjoyed a remarkable career in 12 seasons away with Real Madrid and Juventus.
But the veteran striker's homecoming campaign did not go entirely to plan. He scored 24 goals in all competitions, but United finished sixth in the Premier League, enduring a fifth successive season without silverware and finishing with its lowest points tally since the competition's inception.
In more positive news for United fans, the club is expected to bring in the first signing of the ten Hag era in the coming days, with Feyenoord defender Tyrell Malacia reportedly set to join. United is also reportedly in talks with Barcelona over Frenkie de Jong.
None of that will likely be enough to convince Ronaldo that United is worth staying at, given the lack of a place in the UEFA Champions League and his own advancing years.