Altidore fell to the ground in agony while chasing after a long ball down the line in the 20th minute. After lying on the ground in agony for several minutes, he had to be stretchered from the field at Arena das Dunas.
"We're sorry for Jozy. We hope it's not as bad," head coach Jurgen Klinsmann said. "We hope we're going to get him back as quickly as possible."
"My heart goes out to him because he's been working so hard," forward Clint Dempsey said of Altidore. "In camp, he's been looking really sharp and I know how much he really wanted to do well. You could see tears in his eyes going back to the locker room."
However, he had started to pick up form during the U.S.'s pre-World Cup training camp in California and scored twice in a 2-1 victory over Nigeria in the team's final warm-up game before traveling to Brazil.
After his decent performance as emergency substitute, Johannsson now appears to be in pole position to start the Portugal game.
Alitdore wasn't the only injured American. A leg injury forced Matt Besler out at halftime and Dempsey believed he had broken his nose during what was a physical battle with the Ghanaians.
[Related: Slideshow: U.S. players injured during opening match at World Cup ]
"It will continue to happen to all teams – injuries, problems, issues," Klinsmann said. "That's why you have a roster of 23 players and why they have to understand exactly their roles."