Ronaldo joined Juve from Real Madrid in 2018 but reports emerged last week that he wished to leave the Serie A giants.
Premier League champions City – having missed out on Harry Kane – are front-runners for the 36-year-old's signature in what would be a sensational return to English football.
On Friday, Ronaldo was pictured leaving Juventus training having reportedly said his goodbyes to team-mates.
In Allegri's subsequent news conference to preview Saturday's clash with Empoli, the head coach confirmed Ronaldo was leaving the club.
"Yesterday, Cristiano told me that he no longer has any intention to play for Juventus. For this reason he will not be called up for tomorrow's game," Allegri said.
"Things change, it's a law of life. Juventus remains, which is the most important thing. Cristiano gave his contribution, he made himself available, now he leaves and life goes on.
"Cristiano is to be thanked for what he has done, also as an example to the youngsters. But as I said, we must go on."
The seemingly imminent transfer to City is a move that will greatly anger Manchester United fans, as Ronaldo spent six years with the Red Devils after Alex Ferguson signed him from boyhood club Sporting CP in 2003.
Ronaldo won three Premier League titles and the Champions League with United, while also helping them to an FA Cup, two EFL Cups and the FIFA Club World Cup, before joining Madrid in 2009.
At Madrid, Ronaldo won LaLiga twice and the Champions League four times.
He has remained an ultra-reliable frontman for Juventus, scoring 81 times in 98 Serie A matches since the start of 2018-19, but the Bianconeri have continued to fall short in the Champions League.