Tampa Bay selected Armstrong's contract from Triple-A Durham on Tuesday, Neil Solondz of the Rays Radio Network reports.
The Rays designated lefty reliever Ben Bowden for assignment and placed shortstop Wander Franco (quadriceps) on the 10-day injured list. Before signing with the Rays on a minor-league deal earlier this month, Armstrong made seven appearances in the majors with Miami this season, covering 6.2 innings and giving up eight earned runs on 10 hits and three walks.