US midfielder Weston McKennie has tested positive for coronavirus sending Juventus back into isolation, the Italian champions announced on Wednesday.

McKennie, 22, is the second Juventus player to test positive this week after Portuguese star Cristiano Ronaldo, who failed a test while on national team duty.

"During the checks provided for by the protocol in force, Weston McKennie has tested positive for Covid-19," Juventus said in a statement, they said the team had gone into isolation on Wednesday 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 entire Juventus squad was already isolating after two positive cases among non-playing staff on October 3, a procedure that did not prevent them from training or playing, but prohibits contact with the outside.

Italian health regulations stipulate that the two positive players must remain in self-isolation for at least ten days and test negative before rejoining their teammates.

Under UEFA rules a player must provide evidence that he is no longer sick a week before a European match.

Ronaldo who will miss Juventus's Champions League opener against Dynamo Kiev on October 20, must test negative on October 21 to play against Barcelona a week later.

The Portuguese Football Federation announced on Tuesday that Ronaldo tested positive for coronavirus on Monday, the day after Portugal drew 0-0 with France in Paris and would not be in the squad for Wednesday's UEFA Nations League match against Sweden in Lisbon.

The 35-year-old five-time Ballon d'Or winner was "asymptomatic" and had been placed in isolation after the positive test at the national team's training centre in Oeiras, a suburb of Lisbon.

Ronaldo left the Portuguese team camp near Lisbon on Wednesday to fly back to Northern Italy on his private jet, according to media reports.

Juventus defender Giorgio Chiellini said on Tuesday that he had spoken to Ronaldo.

"He's fine and was sunbathing, he'll be back when he's ready," Chiellini said.

Chiellini said "we footballers know we take risks, and we are ready to take them, trying to minimise them as much as possible with all the necessary precautions.

"We must move forward because we know that football is important economically, and for the social value. In the coming weeks there will be other closures, and I believe football is important to people.

"The real victory is to finish the season, the result is secondary."

He added: "We cannot stop. We are not worried, but clearly we often talk about it among ourselves with the awareness of what lies ahead."