Armstrong accepted an assignment to Triple-A Durham after clearing waivers Tuesday, Neil Solondz of the Rays Radio Network reports.
Armstrong was DFA'd by the Rays on Sunday when Nick Anderson was activated from the 60-day injured list. The right-hander holds a 6.75 ERA, 1.50 WHIP and 44:15 K:BB across 36 innings with Tampa Bay and Baltimore this season, so it's not overly surprising to see him go unclaimed on waivers.