This was confirmed by ESPN FC's Doug McIntyre:
"United States forward Jozy Altidore will miss Sunday's Group G World Cup match versus Portugal due to a strained left hamstring, the U.S. federation confirmed Thursday.
"'He will not be available for Sunday's game, but we are hopeful he will be available in the tournament,' U.S. Soccer spokesman Michael Kammarman said of the 24-year-old.
"The Americans complete the group stage against three-time champion Germany on June 26. Aron Johannsson replaced Altidore after the 21st minute against Ghana. Chris Wondolowski is the other striker in the squad who could join Clint Dempsey in the attack.
"A 31-year-old Californian, Wondolowski led Major League Soccer with 18 regular-season goals in 2010 and 27 in 2012, and tied for the lead with 16 in 2011.
"'He can score from anything; score when you think there's nothing, no chance and next thing you know he scores,' Beckerman said. 'He works his tail off for the guy behind him. He does a lot of things that don't show up on the stat sheet.'
"Johannson, a 23-year-old who was born in Mobile, Alabama, and grew up in Iceland, scored 26 goals last season for AZ Alkmaar in the Netherlands.
"Wondo is more the finisher in the box,' Jones said. 'Aron is more maybe the guy who's good with the ball and tries to go one against one.'
"Coach Jurgen Klinsmann also could opt to shift from the 4-4-2 formation he used in the opener and start five midfielders.
"'He's a big player for us,' midfielder Kyle Beckerman said of Altidore. 'We're extremely bummed for him and for the team, but we bring 23 players, and it's just a chanc for somebody to step up. Hopefully the injury's not too bad and we can see Jozy later in the tournament.'
"Kammarman did not reveal the severity of the strain. The Americans have just three days off between this weekend's game in Manaus and their first-round finale in Recife on June 26.
"Altidore left the Americans' World Cup opener against Ghana after straining his left hamstring in the 21st minute of Monday's 2-1 win against Ghana, leaving his status for the remainder of the tournament in question."
WorldSoccerTalk.com's Caitlin O'Connell also affirms Altidore's absence but says one of the heroes against Ghana, Clint Dempsey, will suit up for the Americans against Portugal:
"The USMNT will face Portugal in Manaus without striker Jozy Altidore who suffered a hamstring injury during USA's 2-1 victory over Ghana last Monday. Altidore was sprinting for a ball and pulled up clutching the back of his thigh.
"He was carried off the pitch looking to be in immense pain and undewent MRI scans at the team's base in Sao Paulo. Altidore previously suffered a hamstring injury in 2011 and missed the Gold Cup finals.
"While the severity of his injury has yet to be released, the United States will likely have to rely on Aron Johannsson, who replaced Altidore in the 21st minute, or Chris Wondolowski.
"Clint Dempsey, who scored the quickest goal in USMNT World Cup history on Monday, suffered a broken nose when he collided with Ghana's John Boye while chasing a high ball. Dempsey's status against Portugal is unaffected and is expected to start with no problems.
"ESPN's Jeremy Schaap reproted that Dempsey is unlikely to wear a mask facing Portugal, probably not wanting to feel uncomfortable in the intense heat of Manaus."
The game against Portugal on Sunday will be at 6:00 p.m. EST.