Aguilar, 32, is batting .221 with five home runs and nine RBIs in 36 games this season, his first in Oakland. He was 1-for-20 in his past seven games.
Aguilar is a career .253 hitter with 114 home runs and 402 RBIs in 10 seasons with six teams. He was an All-Star in 2018 with Milwaukee, slugging a career-high 35 home runs and driving in a career-best 108 runs.
In other roster moves by the A's, third baseman Jonah Bride was recalled from Triple-A Las Vegas, right-hander Paul Blackburn was activated from the injured list and right-hander Garrett Acton was optioned to Triple-A Las Vegas.
Blackburn, who is scheduled to start against the Atlanta Braves on Monday, hasn't pitched since last August, first because of right middle finger inflammation, a cracked fingernail during spring training, a blister in his rehab assignment and a recent paternity leave.
Blackburn, who is entering his seventh season, made a combined six rehab starts for Class A Stockton and Triple-A Las Vegas, going 0-0 with a 7.50 ERA.