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Patrick Vieira believes Cristiano Ronaldo is enduring a "difficult period" at Juventus, and the Turin club need "to understand why he is unhappy."

Ronaldo was substituted for the second consecutive game in Juve's 1-0 win over AC Milan on Sunday, and his replacement, Paulo Dybala, went on to score the winner to put Juve back at the head of Serie A:

Upon being taken off by manager Maurizio Sarri, the Portuguese superstar headed straight down the tunnel, and there were reports he left the Allianz Stadium before the final whistle.

Ronaldo, 34, has scored just one goal in his last five appearances, and Nice manager Vieira, a former Juve player, has said the Old Lady need to get control of the situation, per Sky Sport Italia (h/t Football Italia):

"This shows how difficult it is to be a coach. Ronaldo is of different class, together with [Lionel] Messi and Neymar. They are the image of football today. It's important to understand why he is unhappy. From the outside, it seems like he is going through a difficult period. When a player like him stops scoring, there will be some frustration.

"I am not surprised to see Juventus always in first place, as here the player is at the service of the club, and when things are clear, the player can give his best on the field. At Juventus, the club is the real star."

Ronaldo has been relatively quiet for Juventus so far this term, but his return of five goals and one assist in 10 Italian top-flight appearances is still not bad.

He is a victim of the standards he has set himself in the preceding decade, but there was always likely to come a point when his age started to catch up with him.

The former Manchester United and Real Madrid man should still have a big role to play in 2019-20, though, with Juve aiming to win Serie A again and triumph in the UEFA Champions League.

Old Lady goalkeeper Wojciech Szczesny defended his team-mate after the Milan match, saying it was "normal" for Ronaldo to be angry at being substituted and hinting the Portugal captain has not been at 100 per cent fitness of late, per Perform (h/t AS).

The former Arsenal stopper also predicted Ronaldo "will return in better physical condition and, as he has always done, will make the difference in the decisive phase of the season."

It is true that Ronaldo often finds his best form in the second half of the season. A quick look at his record in the Champions League knockout stages is illustrative of that fact:

Sarri and Juventus will hope the international break gives their star man time to recover and get back to full fitness so he can find his best form again.