Paris Saint-Germain forward Neymar suffered an ankle injury in February, but has since returned with two goals in as many friendlies for Brazil.
Rivaldo, who played 74 times for his country, told the Evening Standard that Neymar can continue his recent form and help Brazil progress through the competition.
"If Neymar plays well in the World Cup, Brazil will be even more favourites but the World Cup is difficult and a lot of things can happen when the knockout stages start," the 46-year-old said. "Theoretically, during the first three games in the group stage it is easier.
"But even if Neymar is 100%, other teams are wanting to win, too. It makes things easier if Neymar is 100 per cent fit because he will boost the players' confidence.
"If Neymar is 100% it will make it easier for Brazil to win matches but that doesn't guarantee that we are going to be champions. Even if Neymar plays well one little slip, the ball goes in and you go home. If Neymar plays well it's always good for the fans and Brazil."
Brazil open their World Cup campaign against Switzerland on Sunday.