The Portugal international - who has expressed a desire to leave the club this summer - did not return to pre-season training at the club's Carrington facilities last Monday due to family reasons.
As well as getting permission to miss the first few days of training, Ronaldo has been allowed to skip the first part of United's pre-season tour, which begins with a friendly against Liverpool in Bangkok on Tuesday.
According to the Daily Mail, the Red Devils will rule the 37-year-old out of the tour completely if he is not available by the end of the week.
The club are set to fly out to Australia after facing Liverpool, and they see little point in Ronaldo travelling if he is not ready in the next seven days.
After their stint in Thailand, Man United will travel to Melbourne to face Melbourne Victory and Crystal Palace before meeting Aston Villa in Perth for a friendly.
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