The Portuguese superstar had been due to turn up at United's Carrington facilities at the beginning of the week, the veteran returning later than other members of the squad due to representing his country in the UEFA Nations League.
However, it was reported on Monday that the 37-year-old had been granted permission to miss his opening sessions of pre-season due to family reasons.
With United flying out on their tour of Thailand and Australia on Friday, the expectation was that Ronaldo would make an appearance before that trip.
According to the Daily Mail, that is becoming increasingly unlikely with the club's star man also failing to attend training on Wednesday.
Ronaldo's absence comes at a time when he is also said to have informed club officials that he wishes to depart the club before the end of the summer transfer window.
The legendary figure is reportedly eager to play in the Champions League next season, something which United cannot offer for 2022-23 and there are no guarantees with regards to the following campaign.
While Ronaldo has 12 months remaining on his contract, United possess the option of extending the player's stay at Old Trafford until 2024.
The official stance of the Red Devils is that Ronaldo remains not for sale, but speculation will persist until Ronaldo links up with the rest of the squad.
Chelsea require a direct replacement for club-record signing Romelu Lukaku, who has been allowed to return to Inter Milan on a season-long loan deal.
In a season where the Belgian scored just 15 times in 44 outings in all competitions, Ronaldo contributed 24 strikes from 38 appearances for United.