The Seahawks later confirmed the injury to The Associated Press. Carpenter was taken off the field during practice Wednesday after being injured in a pass-rush drill, and he had an MRI that night.
There's no timetable for when Carpenter will have surgery or on his recovery time, but ACL injuries usually require about nine months of rehabilitation.
"Yesterday, it was bad. Just the way he went down, the way the injury happened," Breno Giacomini, who'll replace Carpenter as the starting right tackle, told The Associated Press. "That's the way it goes. It's tough for him. James will be back, though. He's a tough kid, so he'll be back and we'll try and help him however we can help him.
Carpenter, whom the Seahawks selected 25th overall in the 2011 NFL Draft, has started in all nine of the team's games this season. His is the second major injury on Seattle's offensive line this week. Rookie right guard John Moffitt, the Seahawks' second draft pick, tore ligaments in his right knee during Sunday's 22-17 victory over the Baltimore Ravens and was lost for the season.
The Seahawks' offensive line has played better of late. After giving up 28 sacks through the first seven games, Seattle surrendered just one sack in each of the past two games against Dallas and Baltimore -- two of the better defenses, statistically, in the NFL. They also got a better push in the run game, averaging 140.5 rushing yards the past two weeks after averaging just 77.7 through the first seven games.
Follow Albert Breer on Twitter @AlbertBreer.
The Associated Press contributed to this report.