Juventus boss Maurizio Sarri has revealed that concerns over a niggling knee injury saw him replace Cristiano Ronaldo during Sunday night's Serie A clash with AC Milan.

Ronaldo was substituted in the 55th minute of the league match, with Paulo Dybala, who ultimately scored the winner, sent onto the field as the Portugal international's replacement.

The 34-year-old was also replaced against Lokomotiv Moscow in the Champions League last week, and he showed his frustration upon leaving the field against Milan.

Sarri has claimed that Ronaldo is not currently "at his best" due to a knee problem, but the head coach has no problem with the forward showing his anger upon being brought off the field.

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

Ronaldo, who reportedly left the stadium three minutes before the end of Sunday's clash, has scored six times in 14 appearances for the Old Lady during the 2019-20 campaign.