Denver Broncos center Dan Koppen, who started 12 games last season, suffered a torn anterior cruciate ligament in his left knee and is out for the season, multiple media outlets reported.
Coach John Fox did not give details of the injury following practice.
"Like all injured players, he will get evaluated," he said. "We will keep you up to speed when we have information."
Koppen was injured when he got caught in a pile up during a workout. He was carted off the field.
"I always cringe," Fox said. "You never like to see players get injured, particularly severely injured. But you understand when you do this long enough, whether you're a player or a coach, that this happens. You know, you get them back ready. And you've got some guys that have worked hard off of injuries last year and it will be no different this year."
Later Sunday, the Broncos announced they have agreed to terms with free-agent center Steve Vallos, pending his physical.
Vallos is a sixth-year player who has seen action in 44 career regular-season games, including nine starts.
Vallos played in two games for Jacksonville last season and made one start.
Koppen, 33, signed with the Broncos last month after J.D. Walton had a setback while recovering from a dislocated ankle. Walton could be back by mid-October, but any further complication could put him out for the entire season.
Koppen, who was released by the New England Patriots following the 2011 season, filled in for Walton last season when Walton was injured.
After Koppen was injured, the Broncos had Manny Ramirez, who is a guard, playing center and snapping the ball to Peyton Manning. That was before they announced the agreement with Vallos.
"We need that flexibility," Fox said. "When you pick your offensive line, not just for the season, but for game day, you have to pick them from the standpoint of having some flexibility. You've got to have a swing guard/center, you've got to have a swing tackle/guard, for the games. Typically you go in with five and bring up two backups. I'm sure that's the way we will go. And who they are, these guys will determine by how they play in this training camp and preseason."