A U.S. Soccer Federation spokesman said Altidore could be out four-to-six weeks after suffering a left hamstring strain in the ninth minute of the United States 2-0 quarterfinal win against Jamaica on June 19 in Washington, D.C.
Altidore had an MRI exam on Tuesday in Houston that confirmed the injury.
The United States is not permitted to replace an injured player on the roster during the competition.
Altidore scored two of the team's four goals during the group stage, including a blast from distance that proved to be the winner in a 1-0 outing against Guadeloupe. The win sent the United States into the quarterfinals.
Juan Agudelo replaced Altidore in the Jamaica match. Agudelo started in the United States' 1-0 win against Panama on Wednesday. He could see another start against Mexico in the championship match.
Dempsey appreciates sea of green
U.S. midfielder Clint Dempsey believes at least some Mexican fans are beginning to appreciate the red, white and blue.
The Americans started the first game of Wednesday's semifinal Gold Cup doubleheader against Panama at Reliant Stadium before a less-than-capacity crowd, but it began to fill toward the end of the match as Mexican fans tickled in for the nightcap between El Tri and El Salvador.
A splatter of red behind one of the goals - comprised of the United States' supporters' group Sam's Army, was eventually engulfed by a sea of green Mexican jerseys.
Many Mexicans cheered when Dempsey scored the decisive goal for the Americans in the 77th minute. Mexico fans even yelled the traditional expletive chant when Panama goalkeeper Jaime Penedo took a goal kick.
"I'd like to think it's two things," Dempsey said. "I'd like to think it's a sign of respect the game is developing more and more in the States and that we are playing a better style of football," he said. "And, two, I'd like to think that a lot of people that were in the stands tonight that were maybe wearing a green jersey are making a living here and they have maybe a little more appreciation for this country."
Mexico commonly draws capacity crowds throughout the United States for friendlies, as well as when it plays in the Gold Cup ever two years. It drew 80,108 in Dallas for its opener against El Salvador, 62,000 in Chicago against Costa Rica and 78,807 for Saturday's quarterfinals at New Meadowlands Stadium.
Wednesday's crowd was 70,627.
"I have a lot of respect for the Hispanics whether it's El Salvador, Mexico, what have you," added Dempsey, who is from Texas. "I grew up playing in Mexican league, so, it's great to see that appreciation for football and also to see that appreciation here for us in being in a stadium that is packed it's nice to see and I think you'll see it again when we play in the final."
Donovan sparks win off the bench
Landon Donovan wasn't in coach Bob Bradley's starting lineup for the second consecutive match.
Alejandro Bedoya started in place of Donovan, who also didn't start in the quarterfinals against Jamaica after arriving only eight hours before the start of that match because he was returning from his twin sister's wedding. The veteran midfielder came in for Sacha Kljestan at halftime of the United States' 1-0 win against Panama.
Bradley said there was nothing physically wrong with Donovan. The move was tactical.
"We talked as a team and we felt we had a good game against Jamaica and it made sense to build on that," Bradley said. "Landon has had a great attitude about what is best for the team and it's a great thing to know that when you bring him on the field he can make a special play."
Donovan helped set up the winning goal when he fed Dempsey with a rolling pass in the 77th minute. All Dempsey had to do is poke in the ball from a few meters away from the goal line.
It is not clear if Bradley will use the same move against Mexico in the Gold Cup final.
Asked if it was difficult to communicate it to Donovan that he would begin on the bench, Bradley said it wasn't a difficult task.
"Landon watched the game the other day and he felt we played well and thought Alejandro Bedoya played well," Bradley said. "There is a sense of what we're in together and in some days, even if the decision doesn't go your way, it's about the team.
"I think we've established that over time. It's been the way it's worked in all the teams that I've coached. Not everybody agrees all the time, that's for sure. Hristo Stoichkov didn't agree with everything I did, but we still talk a lot."
Win against Panama first in Houston for United States
The United States' win Wednesday was its first in Houston. The Americans had played three times in Houston and lost to Ecuador in 1988 and drew with Mexico (0-0) in May 2003 and 2-2 in February 2008.
Salcido, Guardado injured
Mexico defender Carlos Salcido had to leave Mexcio's 2-0 semifinal win against Honduras in the 56th minute with a foot injury. He was replaced by Jorge Torres Nilo.
"I have some pain in the heel," Salcido said. "I have an issue there since the last two games. It has something to do with the training surfaces being too hard. This field was also hard. It began to hurt a lot. They gave me a substance to reduce the game prior to the game but it didn't work that well."
Salcido said he asked to be removed because he wasn't 100 percent and he prefers to give way to a teammate who is healthy. He said he still expects to play against the United States on Saturday.
Guardado, who was replaced by Aldo De Nigris in the 53rd minute, said he might have a similar scenario. Someone else could start in his place if his sprained ankle is not completely healed.
"It's a bit swollen," Guardado said. "We are going to take the necessary measures the following three days. No one wants to miss this game, I obviously want to play, but if I am not at 100 percent I'd have to be honest for the sake of the team. I'll do everything I can to be well."