Armstrong (oblique) has a right side injury that will force him to the injured list to start the season, Greg Johns of MLB.com reports.
While not likely to figure into the closer situation in Seattle, Armstrong was set to be a late-inning setup man for the team. The 28-year-old managed a 1.23 ERA and 0.82 WHIP in 14.2 innings of work last season. He struggled with command this spring -- he surrendered five free passes in seven innings of work -- and this development may offer some explanation for his poor performance.