The Portuguese great has missed the last three days of preseason training as he demands United listen to offers for him.
The Sun says United were initially confident about Ronaldo still being with them for next season.
But directors have now recognised they cannot keep him against his will and want to avoid a long, drawn-out battle.
New United manager Erik ten Hag has also accepted Ronaldo will leave, having hoped to work with the 37 year-old this season.
Despite his age, the super-fit forward still believes he can win honours at the highest level — just not with United.