Portugal, one of the leading contenders coming into the tournament, lost their first game 4-0 to Germany and the United States beat Ghana 2-1.
"Either we win or we start packing our suitcases. This is the scenario we have in front of us ... if we draw we are practically packing our suitcases," Bento told a news conference, alluding to the team's poor goal difference.
Cristiano Ronaldo has been battling a knee injury and Bento did not say whether the talismanic forward would play.
Midfielder Raul Meireles said Ronaldo was fit.
"All the players are feeling very well. We know we have a very difficult game ahead of us," he said.
Portugal will be without central defender Pepe, who was sent off against Germany, and will also be missing injured left back Fabio Coentrao.
Bento said the one doubt was veteran centre back Bruno Alves and if he does not play, he will be replaced by the much more inexperienced Luis Neto.
The coach indicated the team would start the match ready to attack, just as they had done against Germany.
"We have to change the result ... we will go on to the pitch with the same mentality. Just because we lost doesn't mean we have to change anything," he said.
"This is the same team that got us to the World Cup, the same team that did well in the last European Championship. We know what we have to do."
The players will also have to deal with the brutally hot and sticky weather in Manaus, which is in the Amazon jungle.
Asked about the conditions, Bento said both teams would have to deal with the problem.
Pressed again about the challenge of the climate, he replied somewhat waspishly: "It will be difficult to ignore the heat and humidity with so many people talking about the heat and humidity."