The Giants optioned Walker to Triple-A Sacramento on Saturday, Alex Pavlovic of NBC Sports Bay Area reports.
Walker will lose his place in San Francisco's bullpen to make room for Joel Kuhnel, who was claimed off waivers from Milwaukee on Friday. Walker has struggled to a 7.16 ERA and 1.81 WHIP in 32.2 big-league innings this season and could spend the rest of the year at Triple-A.