Whalen was optioned to Triple-A Tacoma on Sunday, Greg Johns of MLB.com reports.
Whalen made his Mariners debut in a starting role Saturday against the Red Sox and failed to impress, surrendering five runs on seven hits and two walks over 5.1 innings. Even if he had performed better, the right-hander likely wouldn't have stuck in the rotation beyond Saturday, as James Paxton (forearm) is on track to return from the disabled list later this week. Reliever Ryne Harper was recalled from Triple-A to take Whalen's spot on the active roster.