Ronaldo was taken off with the Serie A result still in the balance for the second game in succession and the Juve superstar appeared frustrated with the decision.
The 34-year-old forward jogged straight down the tunnel and reportedly left the Allianz Stadium before the final whistle in Turin.
His replacement, Paulo Dybala, later produced an excellent goal to secure Juve a 1-0 victory over struggling Milan.
Ronaldo had underperformed in his 55 minutes on the pitch, leading Sarri to explain the nature of his injury issue.
"He was hit in a training session. He suffered a knee bruise. Maybe the right knee, if I remember correctly," the Bianconeri head coach said.
"It's not a big issue, no twist, only a bruise at the collateral ligament. When he steps he can't push as usual, but I am not worried about it."
Sarri's withdrawal of Ronaldo when three points were up for grabs mirrored the move he made in the midweek Champions League win over Lokomotiv Moscow.
The former Real Madrid man's failure to reappear on the bench attracted criticism from ex-Juve coach Fabio Capello.
"The fact that he didn't sit on the bench and said bad words to Sarri is not a pretty picture," Capello told Sky Sport Italia.
"One must be a champion even when he is replaced, not only when things are going well. Right now, Dybala is in great shape.
"The truth is that Ronaldo has not dribbled past an opponent for three years. I remember it when he went past players and left them there. Now it doesn't happen anymore, while Douglas Costa and Dybala do it.
"If I had been in Sarri's place I would have reproached him in the changing room. I would have told him that he must have respect for his team-mates and if I take him off it is because I want to try to win the game."
Dybala's intervention means Juve remains a point ahead of Serie A title rival Inter going into the international break.
Blaise Matuidi could be forced to pull out of the France squad following further medical assessment on a suspected rib fracture, Sarri confirmed.