Coach Bo Pelini said after Thursday's practice that Martinez hasn't progressed enough in his recovery from an injury to his left big toe.
Armstrong, a redshirt freshman, and fifth-year senior Ron Kellogg III split playing time evenly in the Cornhuskers' 59-20 win against South Dakota State on Sept. 21. The Huskers didn't have a game last week.
Martinez continues to battle turf toe, an injury marked by painful bruising at the base of the toe. Turf toe can become chronic if not allowed to fully heal.
Martinez hasn't played or practiced since the Sept. 14 game against UCLA.