Maurizio Sarri has no qualms with Cristiano Ronaldo's frustrations at being substituted and says Juventus must thank the superstar forward for attempting to play through the pain barrier.

Eyebrows were raised when Ronaldo was withdrawn after 55 minutes of Sunday's 1-0 Serie A win over AC Milan with his replacement Paulo Dybala scoring the winner in Turin.

It came just five days after Ronaldo was substituted with eight minutes remaining against Lokomotiv Moscow and in need of a goal, which eventually arrived from Douglas Costa deep into injury time of the Champions League clash.

Ronaldo gave a sulky reaction on that occasion and headed straight down the tunnel when brought off against struggling Italian rivals Milan.

Juve head coach Sarri again insisted Ronaldo – who reportedly left the Allianz Stadium before the final whistle – is struggling with a knee injury that is affecting his performance and believes his frustrated reactions are a result of that.

"We must thank Ronaldo, because he made a sacrifice to be there at all tonight in a difficult situation," Sarri told Sky Sport Italia.

"He did everything possible to play, but I saw he was not well and thought it best to take him off.

"It's only natural a player is going to be irritated to leave the pitch, especially when he worked so hard to be there. 

"Over the last month, he has had this little knee problem, he had a knock in training and it hurt the collateral ligament. 

"When he trains at high intensity or plays, it unbalances him, so he ends up overcompensating and it damages the calf and thigh muscles.

"It is affecting his performances and he is not at his best at the moment. All players who are trying to give their best will have at least five minutes of being annoyed when they are substituted, but a coach would in general be much more worried if he didn't seem upset."