Armstrong will be out at least a couple weeks with a Grade 1 oblique strain, Greg Johns of MLB.com reports.
We now have an official diagnosis, and it's generally good news as it's the least severe type of strain. Still, Armstrong will be forced to take it slow for a while to avoid more extensive damage. With Anthony Swarzak (shoulder) also on the injured list, the Mariners will have to lean on the likes of Dan Altavilla, Cory Gearrin, Chasen Bradford and Zac Rosscup in the mid-to-late innings.