The New York Giants announced Monday that they’ve placed four players on injured reserve including OL D.J. Fluker (toe), CB Donte Deayon (forearm), LB Curtis Grant (knee) and LB Deontae Skinner (hamstring).
The Giants also signed LB Jeremy Cash to their active roster.
Armstrong, 26, originally signed on with the Rams as an undrafted free agent out of Miami back in 2013. He lasted just over a year in St. Louis before he was waived and later claimed by the Raiders in October of 2014.
The 49ers claimed him off of waivers from the Raiders and played out the final year of his three-year, $1.485 million rookie contract before re-signing with San Francisco last year.
Armstrong later signed a two-year extension with the 49ers in December of last year but was waived last Friday.
In 2017, Armstrong has appeared in 10 games and recorded 54 tackles, one sack, two interceptions, a forced fumble and two pass defenses.