Walton underwent surgery on Monday to replace hardware in his ailing ankle; an infection had apparently caused swelling in the area after the team's minicamp.
Koppen was originally signed by the Broncos to a one-year deal at the $825K minimum last September, just a few weeks before Walton suffered a fracture and disclocation of his left ankle.
If Koppen is brought back for the minimum, his salary will be $940K this season.
The team is hoping to get Walton back onto the practice field sometime in late October or early November, but there is a chance the fourth-year player will be lost for the season.
Walton figures to start the season on the PUP list, which would make him ineligible for Denver's first six games.
After nine seasons with the Patriots, Koppen started 12 games for Denver following Walton's injury, plus the playoff loss to Baltimore. Originally drafted in the fifth round of the 2003 Draft by New England, Koppen will turn 34 in September.
We've updated our free agency tracker to reflect the move.
Updated 1:56pm ET