The physical forward suffered a strained hamstring and was carried off the field in the first half of the Americans' 2-1 victory over Ghana in Natal on Monday.
"We have to see how he reacts [to treatment]," U.S. coach Jurgen Klinsmann said of Altidore. "We're still hopeful that he will be back."
Altidore underwent an MRI at a local hospital on Tuesday after pulling up lame while chasing a long ball just 20 minutes into Monday's tournament opener. He went down in a heap and barely moved before being carted off on a stretcher, visibly in tears. Team USA has not yet received the results of the test.
"I was sprinting and I felt something, and we'll see what happens," Altidore said Monday night. "Of course it was tough for me. I was crushed. I knew right away I couldn't continue, so that was probably the worst feeling."
Altidore got some words of encouragement from Klinsmann before being taken to the locker room.
"As a coach, you give him a positive hug and tell him, 'You'll be back. Stay positive. You'll be back,' " Klinsmann said. "That's what we told him right away in the locker room. That is the big hope that he is coming back."
Altidore's absence would be a considerable loss for the U.S. despite the fact he has struggled to find the back of the net this year. At 6-foot-1, the 24-year-old offers a sizeable presence up front for the Americans. It was hoped he could battle for position and put pressure on the Portugal defense.
Against Ghana, Altidore was replaced by Aron Johannsson, a talented 23-year-old forward who was born in Mobile, Ala., but grew up in Iceland. The AZ Alkmaar star is one of the dual nationals that coach Jurgen Klinsmann brought into the program to bolster depth that has already become essential in this tournament.
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