The 37-year-old has been the centre of transfer speculation after expressing a desire to leave the club in a bid to play Champions League football.
Ronaldo is also yet to feature in pre-season training, with the Portugal international permitted to skip the tour of Thailand and Australia due to family reasons.
With the new season under two weeks away, the Daily Mail are reporting that Ronaldo is set to miss United’s season opener against Brighton & Hove Albion on August 7.
Although the forward has been pictured training in Portugal, his absence in pre-season has left him short of match practice ahead of the new campaign.
While Ronaldo is still yet to return to the club, United’s first-team will play Atletico Madrid on Saturday before the rest of the squad face Rayo Vallecano on Sunday to wrap up the pre-season schedule.