The Portugal star did not train with his team-mates on Tuesday amid reports he has flown home to Madeira as his mother has suffered a stroke.
Juve coach Maurizio Sarri said his availability for the game with Milan at Allianz Stadium is still unknown, while admitting there is "no substitute" for the superstar forward.
"Cristiano is experiencing a personal problem and his availability tomorrow depends on this," head coach Sarri told a news conference.
Thank you for all your messages of support for my mum. She is currently stable and recovering in hospital. Me and my family would like to thank the medical team looking after her, and kindly ask that we are all given some privacy at this time.— Cristiano Ronaldo (@Cristiano) March 3, 2020
"It is clear that the possible absence of Cristiano can change the tactical plans: there is no substitute in the world for him."
Sarri added a further squad update, saying: "We don't have any big problems with regards to injuries but only small ailments: Chiellini went through a personal programme and Gianluigi Buffon trained as normal.
"Sami Khedira is progressing and if he is to take to the field, it will be for a small portion of the game.
"Douglas Costa could also play a part as his condition is improving."
Milan, who drew the home leg 1-1 after Ronaldo scored a last-minute penalty, did not include goalkeeper Gianluigi Donnarumma in their squad list, with the Italy international reportedly out with an ankle injury.
"We are not interested in who Milan have or don't have. If we step out there and prove our worth, we won't have any problems reaching the final," Sarri said.
"You can see they are a solid squad and ready to sacrifice themselves for the collective. Milan are not an easy team to face."