United players who had been away on international duty over the close-season were scheduled to meet up with the rest of the squad, but Ronaldo did not attend just days after the Portugal star reportedly told the club he wants to leave.
His explanation for failing to appear at United's training ground was accepted by the club, but the 37-year-old's absence will be interpreted as the latest move in his bid to engineer an exit from Old Trafford.
Ronaldo is said to be disappointed by United's decline since he returned to the club from Juventus last year, and is keen to play in the Champions League.
Despite the arrival of new manager Ten Hag from Ajax, Ronaldo reportedly does not believe United can challenge for major honours in the near future.
Ronaldo's absence from training comes as United are due to leave for their pre-season tour of Asia and Australia on Friday.
Images from Ten Hag's training session can be seen in the gallery above. To see the next pictures, just swipe.