Niko Kovac's men suffered a controversial 3-1 loss to Brazil in the FIFA World Cup opener and need a win on Wednesday to boost their chances of progressing from Group A.
Jelavic, who is likely to be relegated to the bench as Mario Mandzukic returns from suspension, said his side should earn three points against Cameroon.
"We must forget as soon as possible about the game against Brazil, not talk about it and focus on our game against Cameroon," he told a news conference.
"I didn't watch the Mexico v Cameroon game, only the first part but we are better than Cameroon in every position and will win.
"It makes no sense to talk about Brazil, we must forget that game."
Croatia left-back Danijel Pranjic, who is battling an ankle injury, said the loss to Brazil made no difference for the European nation given they expected to take nothing out of the clash.
He said Kovac's men could not afford a loss at the Arena de Amazonia in Manaus.
"It was expected that maybe we could lose against Brazil. They were not points that we were counting on, although we played a really good match," Pranjic said.
"Now we are ready for the match against Cameroon and we have a good chance against them. We are not allowed to lose so we need to go for three points and need to be prepared for that match."
