Whalen was sent to Triple-A Tacoma on Monday, Shannon Drayer of 710 ESPN Seattle reports.
Whalen was competing for an Opening Day rotation spot but appears to have lost out to Marco Gonzales and Ariel Miranda. He'll return to Tacoma, where he posted a 6.58 ERA in 10 starts last season. He could be back with the Mariners later this season if injuries create a need in the rotation.