The Portuguese superstar is understood to have asked the Red Devils to let him leave this summer, despite only re-signing for the club last summer.
The 37-year-old is said to want to play in the Champions League for the remainder of his career, which Man United won't be doing after finishing sixth last season.
Divisional rivals Chelsea have been heavily linked with a move for Ronaldo, with new Blues owner Todd Boehly meeting the player's agent, Jorge Mendes earlier in the transfer window.
But the Premier League giants remain steadfast in their view that the former Real Madrid and Juventus forward is not for sale, talkSPORT understands.
Despite that, the club have revealed that he is not in the 31-man squad for their pre-season tour and 'has been granted additional time off to deal with a family issue'.
New signing Tyrell Malacia is also on the plane, however, having joined from Feyenoord earlier this week.
There are also a number of youngsters in the squad, including the likes of Alejandro Garnacho, Zidane Iqbal, Facundo Pellistri, Charlie Savage, Tahith Chong, James Garner, and Ethan Laird.
As well as Ronaldo, Phil Jones, Brandon Williams, and Andreas Pereira are also not in the squad.
Jones is currently on a specialist pre-season programme, Williams has returned from his loan spell at Norwich with what is being described as a 'small issue'. while Pereira is getting time off after a loan spell in Brazil with Flamengo.
United's opening match of the tour is against rivals Liverpool on Tuesday in Bangkok.