The Portuguese superstar is understood to have asked to leave the club this summer, with the 37-year-old wanting to play in the Champions League for the remainder of his career.
The club confirmed that the forward is not in their squad for their pre-season games in Thailand, after he told the club he was dealing with a family matter.
Erik ten Hag's side travel to Australia on Wednesday and the Daily Mail report that they have accepted there is no point in Ronaldo joining them if he is not ready by the end of the week.
The Red Devils do hope, however, that Christian Eriksen will join the squad in Australia.
The Dane is expected to sign for the club on a free transfer, after his Brentford contract expired at the end of last season.
Man United are hopeful that he will complete his medical - which is set to be more rigorous than usual after his cardiac arrest last summer - and sign in time to join them in Australia.
But, first, they face Liverpool in Bangkok on Tuesday, where supporters will be hoping to see their only summer signing so far, Tyrell Malacia in action.
The full-back joined from Feyenoord in a £13million deal and could be the first of many new additions this summer.
Both Frenkie de Jong and Lisandro Martinez have been heavily linked with a move to Old Trafford this summer, along with the likes of Anthony and Ruben Neves.