As expected, the Sunderland man will miss Sunday's match against the European nation after suffering a hamstring injury in his country's World Cup opener
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.
U.S. Soccer made the announcement at its training camp in Sao Paulo, where Altidore was on site to receive treatment for his injury. It has not yet been determined whether he will make the long trip to Manaus for the game or remain at the team's base to continue treatment.
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 will play despite receiving a broken nose in the Ghana game and may need to wear a protective mask - in a more advanced position, or choose MLS veteran Chris Wondolowski or 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.