Ronaldo has told the Red Devils to let him leave this summer and, although the club are reluctant to sell, all eyes are on the 37-year-old's next move.
It is understood the legendary goalscorer wants to make the most of his final years at the top level, rather than spending a season competing in the Europa League with United.
This is backed up by Fonte, who insists his Portugal teammate is obsessed with the Champions League and may not be convinced by the current Red Devils squad.
The Lille defender told talkSPORT: "What I do know is that the man loves the Champions League.
"He believes it is the best competition in the world and knows how he has to perform to win Ballons d'Or.
"Talking about Man United, I think they really have a good coach. I've played twice against Ajax and I was really impressed with the way they played and their philosophy.
"He needs to be winning trophies and competing for trophies for the Ballon d'Or.
"He is a serial winner, as everyone knows. He might be looking at the squad and thinking, 'Am I going to be able to win some trophies this year?'
"Maybe he doesn't believe it. I don't know. Only he can say."
Bayern Munich, Chelsea and Napoli have emerged as the main contenders to sign Ronaldo this summer.
However, Fonte suggests his compatriot would be most tempted by a return to European champions Real Madrid.
He said: "Would he go back to Madrid? That would be, for him, the best.
"If they want him back or not, that's a different question."
Meanwhile, links to Bayern have gathered momentum following talk from iconic midfielder Lothar Matthaus.
"Ronaldo to Bayern would be sexy!" Matthaus told Sky Germany. "At least you should think through the options and play through them once.
"If Bayern are of the opinion that Cristiano Ronaldo is still physically able to really help for a year or two and that they can invest the transfer fee generated for Lewandowski more or less like-for-like in the Portuguese megastar, I would think about it."
Referencing those comments, Fonte said: "Bayern Munich would be interesting - because he can win trophies and be competitive in the Champions League."