NFL Network's Ian Rapoport reported that the procedure was a "minor cleanup," and he added that Armstead should be ready for "most, if not all, of the spring drills."
Injuries have plagued Armstead since the Niners took him with the No. 17 overall pick in the first round of the 2015 NFL draft.
Armstead started all six games he played in last season and finished with 16 tackles and 1.5 sacks.
His career has been disappointing overall, as evidenced by his pedestrian production of 50 tackles and 6.0 sacks in 30 games.
Despite that, the Niners picked up the fifth-year option in his contract for 2019, meaning big strides are expected in 2018.
Armstead is one of three first-round picks penciled to start on San Francisco's defensive line this season along with DeForest Buckner and Solomon Thomas.
If Armstead does miss some time, it will allow the likes of Cassius Marsh, Ronald Blair and rookie fourth-round pick Kentavius Street to see more action at defensive end.