Armstrong was traded to the Mariners in exchange for $500,000 of international bonus pool money.
Armstrong appeared in 21 games for the Indians in 2017, accruing a 4.38 ERA and 1.34 WHIP while sporting a 20:10 K:BB in 24.2 innings. The 27-year-old right-hander spent about half his time in Triple-A this past year, but might be able to find a home in Seattle's bullpen and earn a career high in innings and appearances. With Triple-A Columbus, Armstrong posted a 2.42 ERA in 26 innings.