The 37-year-old expressed a desire to leave United if they receive a suitable offer for him, citing the failure to qualify for the Champions League as a factor in his decision.
According to the Daily Mail, Man United were caught off guard by Ronaldo's request, with the Portugal international under contract with the club until next summer.
United were disappointed with the timing of the demand, which has left the Red Devils looking for a potential replacement.
The issue was reportedly addressed by Ten Hag in a team meeting on Monday when the majority of the squad returned to training.
Ronaldo has not returned to training due to family reasons, while the club have confirmed that the forward will miss the pre-season tour to Thailand and Australia.
Man United are scheduled to face Liverpool in Thailand before heading to Australia for friendly games against Melbourne Victory, Crystal Palace and Aston Villa.
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