Cristiano Ronaldo has a big problem with Juventus manager Maurizio Sarri. 

That's according to former Real Madrid and Roma player Antonio Cassano, who has been discussing the situation on Italian TV. 

"I think Cristiano has a big problem with Sarri. It all started with the two consecutive substitutions, that would really irritate a player of his standing," Cassano explained, per AS.

"In my opinion, there's a problem and Juventus have done a brilliant job of keeping it hidden."

It's a pretty big claim to make, but we have seen signs that there's a rift between Ronaldo and Sarri in recent weeks. 

During Juve's win against AC Milan on November 10, the Portuguese superstar was subbed off early in the second half.

He appeared to give Sarri a 'dirty look' on the touchline before storming down the tunnel and reportedly leaving the stadium before full-time. 

That came just a few days after Ronaldo looked unhappy after being hauled off by his manager during a Champions League fixture against Lokomotiv Moscow. 

But both men have since moved to play down any issues, with the forward revealing he was playing through an injury in November and Sarri claiming their relationship is 'good'. 

"My relationship with Cristiano is good, if he gets worked out when he is subbed, it could be positive, he is highly motivated," the Italian coach said.

"I thought the same about Simeone. I've spent many years watching players out on the pitch so, in some ways, I'm not too bothered about their reactions when they're taken off." 

Of course, Sarri's words weren't enough to convince Cassano that all is well at Juventus - instead, he seems to think a storm is brewing in Turin, so the next few weeks will make for interesting viewing.  

And, if Ronaldo misses out on winning the Ballon d'Or, as has been widely reported, it's hard to see his mood improving anytime soon.