Ronaldo – a five-time Ballon d'Or winner – has not claimed the individual award since leaving Madrid for Juve in 2018, with Luka Modric and Lionel Messi honoured in back-to-back seasons.
Ronaldo edged the Liverpool star in the Nations League final to claim the inaugural trophy, but it wasn't enough to topple Van Dijk and Messi, prompting Mendes to highlight the award as an "injustice".
"Cristiano deserved to win the Ballon d'Or at least once in the last two years," Mendes said. "If he had played at Real Madrid, he would have won it, but next year I think he can win it again.
"For me it is an injustice [that Cristiano hasn't won the Ballon d'Or]. He won the Nations League and he is a champion.
"But he knows what he should do and then we will see what happens in 2020."
On the 34-year-old forward, Mendes added: "Cristiano for me is the best player in the history of world football.
"Portugal before Cristiano had won nothing. With Cristiano they have won the European Championship and Nations League, [while] in 2004 they were a finalist in the European Championship too.
"He transformed world football. We are talking about Portugal, a small country that has changed a lot thanks to Cristiano."
Ronaldo has scored 11 goals across all competitions for Italian champions Juventus this season, nine of those coming in Serie A.