Altidore, who started the first game of the past two World Cups for his country, will miss the fixture after straining his left hamstring 20 minutes into Monday’s opening victory over Ghana.
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Altidore’s absence now leaves coach Jurgen Klinsmann with a selection dilemma ahead of the Portugal game, with the Sunderland star one of the more traditional strikers in his team's squad.
The German trainer could either deploy Clint Dempsey - who may have to play with a protective mask after picking up a broken nose in the match against Ghana - in a more advanced position, or choose between MLS veteran Chris Wondolowski and youngster Aron Johansson as a more direct replacement.
Centre-back Matt Besler, meanwhile, who left the opener at halftime with a tight hamstring, is likely to be cleared to play.