On Friday, 49ers head coach Kyle Shanahan said the move was made to make room for the activation of defensive end Tank Carradine.
"We're going to activate Tank, so we were going to be one up, so we had to get one down," Shanahan said during his Friday press conference. "We had five linebackers, which is about one too many, so it's a decision we had to make."
Armstrong spent the past three seasons with San Francisco after originally 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.
"We'd love to keep everybody," Shanahan explained. "Just, the numbers didn't work out for him."
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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