The team promoted Shayne Skov from their practice squad to take Armstrong’s place on their 53-man roster.
Skov appeared in 15 games for the 49ers last season and recorded four tackles, all coming on special teams.
With Armstrong being lost, Michael Wilhoite will likely get more snaps in the defensive rotation for the 49ers. Skov could then take over some of Wilhoite’s workload on special teams.
The 49ers then signed linebacker Curtis Grant to their practice squad to take Skov’s spot.