Altidore was carried off the field midway through the first half of the U.S. opening win Monday in Natal with what was reported to be strained left hamstring.
An MRI performed Tuesday confirmed that finding, U.S. Soccer's Michael Kammarman said. And although Altidore has been ruled out for Sunday, U.S. Soccer's Michael Kammarman said that "we are hopeful that he will be available in the tournament."
Defender Matt Besler, who also came out of Monday's win over Ghana with a hamstring strain, had an MRI, but his condition is much less serious. Besler trained with the rest of the team Thursday and is available for Sunday's game.
When the U.S. meets Portugal in its second group-play match, Portugal will need a win to advance to the second round.