Five-time Ballon d'Or winner Ronaldo returned a positive result while on international duty this week and flew back to Italy ahead of Portugal's Nations League meeting with Sweden.
The 35-year-old travelled on "a medical flight authorised by the competent health authorities".
Ronaldo was reportedly taken home in an ambulance to ensure no quarantine protocols were broken and he will now undergo a period of isolation.
The rest of the Serie A champions' squad that did not leave for international duty have also gone into isolation after McKennie tested positive for coronavirus.
A Juve statement read: "In compliance with the regulations and the protocol, the team entered into fiduciary isolation this evening.
"This procedure will allow all persons negative to the controls to carry out regular training and match activities, but will not allow contact with people outside of the group.
"The club is in constant contact with the competent health authorities."
Juve announced on Tuesday that the Under-23 squad had begun isolating after Lamberto Zauli tested positive, with Luca Rosa and another member of staff having been found to have contracted the virus 24 hours later.
Andrea Pirlo's side are due to travel to Crotone on Saturday before getting their Champions League campaign under way at Dynamo Kiev on Tuesday.