ENGLEWOOD, Colo. -- Broncos center Dan Koppen is out for the season after tearing his left ACL on Sunday in 9-on-7 drills, less than six weeks after rejoining Denver following J.D. Walton's setback in his comeback from ankle surgery.
Koppen, an 11-year veteran who played his first nine seasons with the Patriots, was caught in a pile early in the workout and was carted off the field and taken for an MRI, which revealed the season-ending injury.
Koppen's injury had earlier been reported by The Denver Post.
Manny Ramirez, who took most of the snaps at center during Denver's offseason practices, stepped in for Koppen, whose teams have gone 108-28 in games he has started.
"I always cringe," a somber-sounding coach John Fox said after practice. "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."
Koppen's injury is the latest setback to Peyton Manning's patched-up pocket of protection.
Left guard Zane Beadles was the only starting lineman to come out of last season unscathed.
All-Pro left tackle Ryan Clady has been limited at training camp since signing a $57.5 million contract earlier this month as he recovers from right shoulder surgery. Tackle Orlando Franklin (toe, shoulder) and guard Chris Kuper (ankle) are also coming off surgeries. Franklin has been back at practice but Kuper hasn't.
With Kuper needing another operation this offseason, the Broncos signed right guard Louis Vasquez in free agency.
The Broncos also added free-agent center Steve Vallos Sunday night, pending a physical. Vallos is a six-year veteran who has played in Seattle, Cleveland, Philadelphia and Jacksonville with nine career starts.
Ramirez started 11 games at right guard last season.
"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."
Defensive lineman Derek Wolfe also sprained an ankle and didn't return to Saturday's practice, which was the second straight workout in full pads for the Broncos.
"That's the Catch-22 any coach goes through any time of year," Fox said. "You're getting ready for the season and you need to get into pads because you've got to get in shape and yet you're still trying to make sure you've got enough guys to battle with."
Information from The Associated Press was used in this report.