Armstrong (calf) threw a bullpen session Friday, Tim Stebbins of MLB.com reports.
Armstrong was recently cleared to begin a throwing progression, a good indication that he is recovering from a left calf strain that landed him on the injured list in late July. He was transferred to the 60-day IL on Aug. 4, but he could be back on the Guardians' major-league roster right before the end of the regular season in mid-to-late September.