Cristiano Ronaldo's fitness and attitude has come under the spotlight after he reacted angrily to being substituted during Juventus' win over AC Milan and coach Maurizio Sarri said the 34-year-old had been nursing a knee injury for some time.

Ronaldo now faces two matches with Portugal where he will want to help the European champions guarantee their place at Euro 2020 and close in on the 100-goal milestone for his country.

He currently has 95 goals and, given his prolific record, it is not inconceivable that he could reach 100 against Lithuania and Luxemburg -- if he can play.

Ronaldo gave Sarri a dirty look and walked straight down the tunnel after being substituted in the 55th minute on Sunday, prompting widespread criticism led by former Real Madrid, Juventus and England coach Fabio Capello.

"Ronaldo hasn't dribbled past a player in three years and I remember when he could leave them on the floor," Capello told Sky Sport Italia. "Sarri was right to take him off. The fact that he did not go and sit on the bench was not a good image."

Sarri said the player had taken a knock on the knee in training a month ago was still feeling the effects.

"When he trains with high intensity and plays matches, he feels some pain. He can't train too hard and has difficulty when he kicks the ball."

Sarri added that Ronaldo was also suffering problems with his thigh and calf, raising questions as to whether the striker was taking unnecessary risks by playing on rather than taking some time out to regain full fitness.

Ronaldo's career has been remarkably injury-free and his longest layoff was in 2008 when a fractured knee cap sidelined him for 94 days, during which he missed 11 games.

An ankle injury cost him nine games in 2009 and he suffered another knee injury during the Euro 2016 final for Portugal against France.

He was also troubled by tendinosis around his left knee in the run-up to the 2014 World Cup. He played in all of Portugal's games but later admitted that he had taken a risk by playing through it.

According to the specialist website Transfermarkt, he has only missed 53 games in his entire career due to injury.

Sarri described Ronaldo's angry reaction to being substituted as normal for a top player, but he will probably be more than happy to let Portugal coach Fernando Santos deal with the problem for the next week.