(AP) - The Seattle Seahawks will go into their showdown against New Orleans on Monday night short-handed in the secondary after starting cornerback Walter Thurmond was officially suspended Tuesday by the NFL for the team's next four games for violating the league's substance abuse policy. Thurmond will be eligible to return to the active roster on Monday, Dec. 23, before the final game of the regular season, but his absence leaves a hole in a secondary already missing starting cornerback Brandon Browner because of a groin injury. Thurmond's suspension was first reported by NFL.com over the weekend. Thurmond took to Twitter and apologized on Sunday, writing, "Yes I made a mistake and I have to live with the consequences. I'm disappointed in myself for letting my teammates and family down. Thurmond has started three games this season for Seattle, including the team's last game against Minnesota when he had his first interception of the season and returned it 29 yards for a touchdown. Even when he wasn't starting, Thurmond was a critical member of one of the NFL's best secondary units as the fifth defensive back in passing situations. Thurmond beat out veteran Antoine Winfield, whom Seattle signed in the offseason, for the nickel cornerback spot during training camp. He had struggled with injuries throughout his career but was finally healthy and playing well ahead of an offseason where he will be a free agent. Seattle moved quickly to fill Thurmond's roster spot, signing veteran Perrish Cox, who worked out for the team recently after being released by San Francisco on Nov. 12. Cox played in 25 games over two seasons with the 49ers in a reserve role after starting nine games for Denver in 2010. Seattle had also worked out former cornerback Marcus Trufant, who has not played this season, before deciding on signing Cox. With Thurmond and Browner out, the Seahawks are likely to lean on young backups Byron Maxwell and Jeremy Lane. Maxwell had been seeing more playing time, especially on passing downs when Thurmond would move inside to cover slot receivers. Lane started three games late last season when Browner was serving a four-game suspension for using performance enhancing drugs and Thurmond was injured.