ENGLEWOOD, Colo. --With two more transactions Tuesday -- making a total of 13 since Sunday -- the Broncos reached the NFL-mandated, 75-player roster.

Third-year safety Quinton Carter was placed on injured reserve, while fourth-year center J.D. Walton has been placed on the Reserve/Physically Unable to Perform list.

The Broncos knew since mid-June that Walton would be out until mid-October after undergoing an additional procedure on the injured ankle that sent him to injured reserve after the first four games in 2012.

The reserve/PUP designation means that Walton may not practice with the club through the first six weeks of the regular season. Beginning Oct. 15, the center is eligible to practice for up to a 21-day period between then and Nov. 19.

At any point during that practice window, Walton -- who started 36 consecutive games to begin his career before suffering the ankle injury -- can return to the active roster.

Meanwhile, Carter will not play in the 2013 season. He was placed on injured reserve and was not given the designation to return.

Carter was also placed on injured reserve with a knee injury last season after playing just three games on defense and special teams. As a rookie in 2011, Carter played in all 16 games -- starting 10 -- and even snared an interception in each of the club's two postseason games that year as a starter. He became just the ninth rookie in the Super Bowl era to post an interception in consecutive playoff contests.

The club plays its preseason finale against the Arizona Cardinals at Sports Authority Field at Mile High on Thursday. Two days later, the roster must be trimmed to the final, 53-man squad.

"It's always tough," wide receiver Eric Decker said of roster cutdowns. "I'm going into my fourth year. You build a relationship with everybody in this locker room. To see a guy go, whether it's moving to a different team or getting cut in general, it's always tough."