The decision to waive Armstrong was made because the 49ers needed a roster spot to activate defensive end Tank Carradine off of injured reserve, which they did on Saturday.
"We had five linebackers, which is about one too many, so it's a decision we had to make," 49ers head coach Kyle Shanahan said on Friday.
Armstrong spent the past three seasons with San Francisco after initially being claimed by the team in late-November of 2015. He has appeared in all 10 games this season and started five while compiling 54 tackles, a sack, two passes defensed, two interceptions, and a forced fumble.
In December of 2016, Armstrong signed a two-year extension with the 49ers that would have kept him with the team through the 2018 season. He is in his fifth NFL season and spent time with the St. Louis Rams and Oakland Raiders before joining the 49ers in 2015.
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