Armstrong was recalled from Triple-A Columbus on Tuesday, Zack Meisel of Cleveland.com reports.
The Indians had an extra roster spot after placing Austin Jackson (toe) on the disabled list, so they opted to bolster the bullpen by bringing Armstrong back to Cleveland. The 26-year-old struggled a fair amount in his first stint with the big-league squad this season, so he'll likely serve as a middle-relief option until Jackson is ready to return to action.