Armstrong was designated for assignment by the Mariners after Thursday's 14-2 win over the Rangers, Greg Johns of MLB.com reports.
Armstrong spent the first couple weeks of the season on the injured list with an oblique injury and allowed six runs on eight hits over 3.2 innings since being activated April 12. The 28-year-old gave up only two runs and had a 15:3 K:BB over 14.2 innings with the Mariners last season and has a 3.65 ERA and 1.25 WHIP across 61.2 major-league innings for his career.