Valdez was called up from Triple-A Fresno on Tuesday, Chandler Rome of the Houston Chronicle reports.
Valdez will swap places with fellow left-hander Cionel Perez, who was optioned to the minors in a corresponding move. The 24-year-old Valdez was scheduled to start for the Grizzlies on Tuesday, so he should be available to pitch multiple innings out of the bullpen for the Astros. He owns a 4.11 ERA across 103 innings between Double- and Triple-A this season.