The 36-year-old arrived at training on Friday morning, but stayed for just 40 minutes before he was spotted leaving.
Ronaldo, who has a year left on his contract, has reportedly asked his club to let him leave and both the player and agent, Jorge Mendes, are waiting for the Premier League side to make an official bid.
In Italy he is said to be on a massive salary of €31million net per season but is unlikely to get that in Manchester.
Any move to the Etihad will cause anger at Old Trafford where he became one of the best players in the world under Sir Alex Ferguson at Man United.
Having joined the club as a teenager in 2003, he scored 118 goals across six seasons before signing for Real Madrid in an £80m deal.
According to Spanish publication, AS, only a transfer fee agreement stands in the way of the deal being done and it is claimed Juventus want 'at least €25million' or Gabriel Jesus as part of a swap deal.
However, manager Pep Guardiola isn't keen on letting Jesus leave this summer after his bright start to the season in a new wider position.
Instead, AS claim City have offered Raheem Sterling to Juventus, with negotiations set to carry into the final days of the summer transfer window.
Should Sterling make way then the no.7 shirt will be vacated - a number synonymous with Ronaldo, stretching back to his United days.
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City had been heavily linked with Harry Kane, but the striker will be remaining at Tottenham and former Chelsea and England midfielder Joe Cole thinks Ronaldo is a better signing.
"I backed Chelsea for the title and I said the only way I'd stick an asterisks on that is if Harry Kane went to Man City - but I think Ronaldo is an even better option for them.
"Financially, commercially and most importantly you've got to remember he outscored Romelu Lukau in Serie A last year - he's not a normal 36 year old!
"I think he'll feel no way about going to Manchester City. I think he's just so driven.
"His allegiances to Manchester United are there, but I don't think he'll be thinking, 'I'm going to ruin my relationship with the Manchester United supporters or the club'".