Armstrong will open the second half of Saturday's doubleheader against the Royals, Ryan Bass of Bally Sports Sun reports.
The Rays may be playing two games in one day, but they only had to use two members of a well-rested bullpen in the afternoon contest, so Armstrong won't face much pressure to eat innings. He did throw three innings as an opener in his final appearance before the break, but he hasn't lasted more than two frames in any of his other 12 outings. Cooper Criswell is expected to follow Armstrong as the bulk reliever.