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.