Armstrong will serve as the Guardians' opener Friday in Game 4 of the ALCS against the Rays, Tim Stebbins of MLB.com reports.
Armstrong has been terrific this postseason, logging 5.1 scoreless innings while allowing just one hit with a 5:0 K:BB. Tanner Bibee and/or Joey Cantillo will be options for bulk relief for the Guardians on Friday.