Armstrign (back) began a throwing program at the alternate training site Wednesday, Roch Kubatko of MASNSports.com reports.
The 30-year-old is already eligible to return from the 10-day injured list, but he's unlikely to be activated for another week or two while he rebuilds his arm strength. Armstrong went down with back soreness in late August, but manager Brandon Hyde expected him to rejoin the team by the end of the regular season.