Whalen was placed on the restricted list at Triple-A Tacoma in order to deal with "personal issues," Bob Dutton of The News Tribune reports.
Whalen has been struggling with the Rainiers, going 0-7 with a 6.58 ERA and 1.53 WHIP in 53.1 innings across 10 starts. The 23-year-old right-hander's return date is currently unknown due to the nature of his absence, and his departure opens up another spot on the Mariners' 40-man roster for the time being.