The 37-year-old has made it clear he is keen to leave Old Trafford as he seeks Champions League football.
The Portuguese talisman has been on holiday with his partner Georgina in Mallorca, and is hopeful United don't stand in his way should they receive a satisfactory offer for him.
He was expected to report to the Red Devils' Carrington training base on Monday, alongside those players who were on international duty last month, but will now be absent, citing family reasons, according to reports.
Ronaldo has been linked with several clubs, with Chelsea, Bayern Munich, Paris Saint-Germain, and even Napoli being touted at potential destinations.
Roma, along with Sporting Lisbon and Porto are other names who have also been credited with an interest.
United remain adamant the five-time Ballon d'Or winner is not for sale, and are prepared to play hardball, given he has one year left on his current contract.
After netting 24 goals in 38 games last campaign, including 18 in the Premier League, he was the third-highest scorer in the English top flight, showing he still has what it takes at the top.
Ronaldo is expected to fly to the Thailand later this week on a 16-day tour with the rest of his teammates, and then travel to Australia on Friday for the final leg of their trip.
Ten Hag has impressed his playing staff so far due to his thorough training regime, but does face a tough test to persuade Ronaldo to stay put.
Ronaldo has also made it clear he wants to continue challenging for trophies and believes United are not capable of doing so.
The club have also been slow off the mark in the transfer window, which is likely to have also played a part in his decision to want out.
Full-back Tyrell Malacia is expected to be the first addition to Ten Hag's side joining from Feyenoord, and the club remain hopeful more recruits will follow.
United were keen on Uruguayan striker Darwin Nunez before Liverpool made their move, and have so far been priced out of a move for Ajax's £70million-rated Brazil star Antony.
Ronaldo played his last match for Portugal on June 9 so has been off for three weeks, and is now set to return to training.
Fellow countryman and teammate Bruno Fernandes confirmed last week himself will return on Monday, and was expecting to see Ronaldo there.
"What I hope is that Cristiano will be there with me on Monday. I don't believe the club is willing to lose a player like him," Fernandes said.
Ronaldo is set to miss out but will no doubt return imminently.