Right-hander Cody Martin was recalled from Triple-A Tacoma before Saturday's game against the Cubs.
Chicago scored five runs in two innings off Karns in Friday's 12-1 romp. He gave up three hits, walked three and allowed a home run to David Ross.
Karns began the season as a starter, but was moved to the bullpen in June. He has a 5.15 ERA.
Martin has appeared in two games and thrown four innings for Seattle this season, allowing one run and five hits.