Armstrong (groin) is scheduled to begin a rehab assignment Tuesday with Double-A Akron, Tim Stebbins of MLB.com reports.
The Guardians are planning for Armstrong to cover one inning and around 20-to-25 pitches in the appearance, which will be his first since April 21. Armstrong is working his way back from a right groin strain and probably won't need to throw more than a couple of innings on the farm before the Guardians feel comfortable bringing him back from the 15-day injured list.