Cristiano Ronaldo reportedly left the Allianz Stadium before the final whistle of Juventus' 1-0 Serie A win over AC Milan on Sunday after being substituted for the second match in a week.

Paulo Dybala replaced Ronaldo on 55 minutes with the Argentina striker going on to score the winning goal 22 minutes later to keep Juventus top in Serie A ahead of Inter Milan.

Five-time Ballon d'Or winner Ronaldo looked glum as he came off and directed some words at coach Maurizio Sarri before heading to the changing rooms and Sky Sport Italia reported that the 34-year-old had left the stadium three minutes before the match finished.

Sarri, however, said he did not know whether Ronaldo had left the stadium early.

"Disrespectful to his teammates? Like I said, I don't know," Sarri said. "If it's true that he left before the end, it is a problem to be resolved with his teammates."

"I have no problem with Cristiano, he should be thanked because he made himself available despite not being in the best condition," said Sarri.

"In the last month, he has had discomfort in his knee and this is a little problem for him."

Ronaldo had reacted furiously after been substituted in his team's 2-1 win against Lokomotiv Moscow midweek which saw Juventus advance to the Champions League last 16.

"The important thing is that he made himself available. If he gets angry when he comes out, this is part of the game," continued the Juventus coach.

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

"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."

Dybala keeps Juventus at Serie A summit

Argentine forward Dybala hit the winner in the 77th minute in the Allianz Stadium. Defending champions Juventus maintain their one-point lead over Antonio Conte's Inter Milan who had taken pole position after their 2-1 win over Verona on Saturday.

AC Milan's seventh defeat in 12 games saw Stefano Pioli's side drop to 14th position, just four points above the relegation zone.

However, Milan had threatened as the former European giants chased their first win against Juventus in Turin in eight years.

Wojciech Szczesny was kept busy in the Juventus goal, denying fellow Pole Krzysztof Piatek, Lucas Paqueta and Hakan Calhanoglu.

Blaise Matuidi missed Juventus's best chance before Dybala's winner when his powerful curling effort flew wide just after the break.

Behind the leaders, Lazio and Cagliari moved into the top four after goal-packed wins.

Lazio saw off newly-promoted Lecce 4-2, while surprise package Cagliari continued their superb form with a 5-2 rout of Fiorentina.

The two teams are level on 24 points, eight behind Juventus with Lazio ahead of the Sardinians on goal difference.