The Athletics optioned Koenig to Triple-A Las Vegas on Sunday, Martin Gallegos of MLB.com reports.
Oakland hasn't appointed an immediate replacement in the rotation for Koenig, who turned in a 6.38 ERA, 1.75 WHIP and 10:11 K:BB across 18.1 innings in his four starts with the big club following his promotion from Las Vegas earlier this month. Reliever Sam Selman was called up from Triple-A in a corresponding move.