The Mariners optioned Simpson to Triple-A Tacoma on Wednesday, Shannon Drayer of SeattleSports.com reports.
Seattle called up lefty Jhonathan Diaz from Triple-A in a corresponding move, giving the Mariners a fresh arm in the bullpen. Simpson was unlikely to be available for another day or two after he needed 33 pitches to record two outs in his appearance during Tuesday's 22-0 loss to the Brewers. He gave up five runs on three hits and two walks in the outing, inflating his ERA to 9.82 and his WHIP to 2.05 over 7.1 big-league innings on the season.