ALAMEDA, Calif. - The Oakland Raiders placed starting middle linebacker Nick Roach on season-ending injured reserve Wednesday after he was unable to recover from a concussion in the preseason.

Roach originally got hurt Aug. 22 in the third exhibition game against Green Bay. He returned to practice Sept. 3 on a limited basis but only took part in individual drills and he was never able to be cleared for contact. Now he is done for the season.

"It's certainly a blow, no question about it," interim coach Tony Sparano said. "Nick was one of our defensive leaders. He's a guy that we rely an awful lot on. It's one of the body blows that goes on in our business."

Roach was supposed to be the quarterback of the defence and his absence has played a big role in Oakland's early struggles. After playing every defensive snap in his first year in Oakland in 2013, Roach has been unable to make it on the field this year.

After consulting with doctors, Sparano said the team had no indication of when Roach would be cleared to play, leading to the decision to shut him down.

"''It was certainly going to take a little while longer and it was open ended at the end of it," Sparano said.

The Raiders (0-4) have been plagued by miscommunication and alignment issues on defence without their signal caller. Oakland has the second-worst run defence in the NFL through four games with Miles Burris and Kaluka Maiava sharing the duty at middle linebacker.

Roach is Oakland's second linebacker to be shut down for the season this week. Maiava was placed on injured reserve Tuesday with a hamstring injury, leaving Burris as the starting middle linebacker on Sunday against San Diego.

"I bounce things off Nick all the time and I still will," Burris said. "He's still going to be around. He'll still be just as big a part of this team because he'll be helping everybody out. Nick sees football. He has a great football mind. He sees things that a lot of people can't see on the field so that will be big."

The Raiders got some positive news at the position with outside linebacker Sio Moore back at practice Wednesday after missing the past two games with an ankle injury.

Moore was limited at practice but his return will provide a boost to the defence.

"When you're not out there fighting every day, you feel sick to your stomach," Moore said. "I just wanted to be out there doing my part to help, doing everything I can."

The Raiders signed Jamar Chaney on Wednesday to replace Roach on the roster. Chaney is in his fifth season in the NFL and was cut in training camp this summer by Denver. He started 23 games from 2010-12 in Philadelphia, including all 16 in 2011, and played one game last season with Atlanta.

Chaney has 161 career tackles with one sack, three interceptions, 12 passes defenced and one forced fumble.

The Raiders had claimed linebacker Ray-Ray Armstrong off waivers from St. Louis on Tuesday. Armstrong joined St. Louis as an undrafted free agent out of Miami in 2013 and played all 16 games as a rookie. Armstrong has played four games this season and has been almost exclusively used on special teams.

NOTES: QB Derek Carr was limited at practice after hurting his left knee and ankle before the bye but is confident he will be able to play Sunday. ... RT Khalif Barnes (quadriceps) did not practice and Menelik Watson may need to start in his place Sunday.

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