Manager Maurizio Sarri took Ronaldo off for the second game in a row, and the 34-year-old responded by giving his boss a "dirty look" and then muttered something before walking "straight down the tunnel," according to Reuters (h/t the Guardian).
Sky Sport Italia (h/t Football Italia) report Ronaldo then left the stadium three minutes before the end of the match in Turin.
Sarri spoke to Sky Sport Italia (h/t Football Italia) after the match about his decision to take his star man off so early on in the game.
"We must thank Ronaldo, because he made a sacrifice to be there at all tonight in a difficult situation. 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's replacement, Paulo Dybala, scored the only goal of the game on 77 minutes to keep the Italian champions top of the table ahead of Inter Milan heading into the international break.
Match of the Day presenter Gary Lineker offered his thoughts on the incident:
Sarri also substituted Ronaldo in the 82nd minute of Wednesday's 2-1 win over Lokomotiv Moscow in the UEFA Champions League. His decision also produced an angry reaction from 34-year-old.
The Juventus boss told reporters after the match that he chose to replace Ronaldo because of some minor injury problems which made him fear he "would hurt himself."
Ronaldo has now been substituted as many times this season as he was in the whole of last campaign, according to Squawka Football.
The forward has started 10 of Juventus' 12 Serie A games this season and all of their Champions League matches, scoring six goals along the way for Sarri's side.
Yet Bleacher Report's Gianni Verschueren says he's not been himself recently:
Ronaldo will now take a break from club action because of the international break. He has been included in the Portugal squad for Euro 2020 qualifiers against Luxembourg and Lithuania.