Criswell was diagnosed with a Grade 1-plus pectoral strain Tuesday, Daniel Kramer of MLB.com reports.
Criswell initially landed on the injured list last Tuesday with a shoulder strain, but further testing determined the issue to be in his pec. He's scheduled to undergo another MRI in 3-to-4 weeks, at which point he may be cleared to begin a throwing program. If all goes according to plan, the Mariners expect to activate the 29-year-old reliever sometime around the Aug. 3 trade deadline.