The Marlins designated Armstrong for assignment Monday, Jordan McPherson of the Miami Herald reports.
Zach Pop was optioned to Triple-A Jacksonville while Armstrong was removed from the 40-man roster entirely as the Marlins reduced their active roster from 28 to 26 men ahead of Monday's deadline. After winning a spot in the Marlins' Opening Day bullpen, Armstrong made seven appearances and was touched up for 10 runs (eight earned) on 10 hits and three walks across 6.2 innings.