Messi and Ronaldo have dominated football's top individual awards for the past decade and are expected to continue that trend this year after outstanding campaigns for Barcelona and Real Madrid, respectively, reports Xinhua news agency.
According to Mourinho, both Portugal and Argentina would be out of contention to win football's showpiece tournament in Russia if not for the two forwards.
"Portugal have an interesting team. Without Ronaldo it would be impossible, but with him nothing is impossible," the Portuguese told ESPN Brasil.
"I think Argentina without Lionel Messi wouldn't be a candidate. But with him, they are one of the favorites."
Mourinho had words of praise for Brazil, who he said have been transformed since Tite replaced Dunga as national coach in June 2016.
"I really like Brazil's basic structure, their tactics, their mentality," the former Porto, Chelsea, Inter Milan and Real Madrid manager said. "There is a mix of natural Brazilian talent with a serious focus on physicality and tactics.
"They have a team that can defend well, concede few goals and have good defensive midfielders. In attack they have Willian, Neymar, Philippe Coutinho and Gabriel Jesus. All are players of outstanding quality."