The Portugal sensation, 37, wants to leave the Red Devils in pursuit of Champions League football, but the club remain reluctant to sell.
Having missed the pre-season tour of Asia and Australia for 'family reasons', Ronaldo returned to action with a 45-minute cameo in an Old Trafford friendly against Rayo Vallecana on Sunday.
The superstar forward left before the end of the game, snubbing Erik ten Hag's post-match team talk, and the United boss has since admitted that behaviour is 'unacceptable'.
Ronaldo is not expected to start Sunday's Premier League opener against Brighton on the bench - due to a combination of the transfer saga and a lack of match fitness.
He has not played more than 45 minutes for United since the 4-0 defeat at Brighton in May.
It is understood Ten Hag has split his squad into two groups in training: those who are match ready and those who need to boost their fitness before the new campaign.
Ronaldo is thought to be in the latter group and on Thursday he posted two images from training with the caption 'step by step'.
talkSPORT understands the striker and his agent Jorge Mendes are still struggling to find a suitable offer from an alternative club.
Transfer reporter Alex Crook said: "The latest is that clubs are not queuing up to sign Ronaldo as he and Mendes expected.
"I do believe the version coming out of his camp, I don't think this is about money, I think he wants to play in the Champions League.
"Egotistically, I don't think he can stomach that Lionel Messi will be playing Champions League football and he'll be in the Europa League.
"But, at the moment, he doesn't have an escape route. I was told Sporting Lisbon on loan was a possibility, but that hasn't kicked on.
"Apart from that, the likes of PSG, Bayern Munich, Chelsea - clubs he believed would be interested - are not coming to the table."