Collingwood have suffered yet another injury with key defender Ben Reid ruled out for at least a month after undergoing surgery on both knees.

It adds to the Pies' woes down back after the club revealed on Monday that fullback Lynden Dunn suffered an ACL injury against the Suns that will sideline him for the remainder of the season.

Reid has been struggling with injuries this season, mostly dealing with calf soreness, until knee concerns recently surfaced. The club decided to conduct surgery in order to have the 29-year-old ready for finals.

The All-Australian backman has played six games this season and was last seen during the club's round 10 win over the Bulldogs.

"He had a scrape on both knees under surgery Monday and we'd expect that will fix the problem," head of football Geoff Walsh told the club's website.

"It's probably a four-week job before he can get back into contention for full training and selection."

Reid spent some time in the forward prior to his injury this season, but will focus on defence when he makes his return to fill the void left by Dunn.

Fortunately for the Pies however, Darcy Moore will return to the VFL this weekend and Jamie Elliott is only a week away from his return to footy.