Germany's 2-2 draw with Ghana on Saturday means the Americans will qualify for the second round if they beat the Portuguese, who are bottom of the group with no points.
"We want to put Cristiano and his team in place. We want to get out there with all the energy that we have, with all the discipline that we're going to bring, with all the aggression we're going to bring to our play and make it our game," he said.
The U.S. will be without forward Jozy Altidore, who left the Ghana game early with a hamstring injury. He will most be likely be replaced by Aron Johannsson.
Although the Americans had several close shaves against Ghana, Klinsmann denied his team had been lucky.
"There is no perfect game against anybody in the World Cup, so you will always give chances away, you will always make some mistakes at a certain point," he told a news conference.
Ghana's draw with Germany showed how well the United States had done to beat the African side, he added.
"For us it's a huge opportunity tomorrow (Sunday) here in Manaus and we will definitely go for it," he said.
Portugal captain Ronaldo has been struggling with a knee problem but looks set to play and will be looking to improve upon a disappointing performance in the 4-0 defeat by Germany.
Goalkeeper Tim Howard said it would be a mistake to focus too much attention on the World Player of the Year.
"If you pay too much attention to Ronaldo someone else will beat us... we have to know where he is," he said.
Howard also said the U.S. team had not trained specially for the heat and humidity in Manaus, which is in the middle of the Amazon jungle.
"We're fit and we're conditioned and as long as we get good sleep and stay hydrated we should be fine," he said.