The Sunderland striker hurt his hamstring in a 2-1 win over Ghana on Monday and appears likely to miss Sunday's Group G match in Manaus.
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"We only talk now about how we can beat this Portuguese team with all these amazing players they have," the German told reporters.
"This is our goal and we believe in it and believe we can go to Manaus and beat them. We want to get in the next round and, if it takes beating Portugal, then this is what we have to do."
Klinsmann was unwilling to reveal his plans to cope with the injured Altidore should the 24-year-old be confirmed unfit.
"I don't want to go too deep into details because obviously we want Portugal to guess a little bit as well but, when one of your key players is not available, does it change certain things? Absolutely it does," he said.
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"We still can field 11 and we are not a man down. We have somebody coming in and we will make it very difficult for Portugal in Manaus and find solutions."
Klinsmann feels that goal difference could be key after Germany thrashed Portugal 4-0 in their opening game but will not go into his team's second match looking for a draw.
"We have very good players in this squad and we have the confidence to go into that game and say 'we are here and want to beat you and get into the next round'," he declared.
"Our approach is not to go to Manaus and defend for 1-1 or 0-0 or whatever it is but we go there and want to win this game."