SANTA CLARA -- The 49ers placed Glenn Dorsey on injured reserve and filled his roster spot with linebacker Ray-Ray Armstrong, whom they claimed off of waivers from the Raiders.
Armstrong, 24, started the season as the Raiders starting linebacker. But he lost that role after two games, then was the focus of a bizarre investigation in Pittsburgh in which he was accused of taunting a police dog, a third-degree felony, before a game against the Steelers. He was cleared last week.
Armstrong (6-3, 220) had 21 tackles and a sack for the Raiders this season. Dorsey, meanwhile, suffered an ACL tear on the 49ers' first defensive snap on Sunday. His spot will be filled by rookie Arik Armstead.