Kiermaier had been out since April 5 with a strained left quadriceps. He took live at-bats against taxi squad pitcher David Hess prior to Saturday's game and was deemed ready. He was available off the bench, with Brett Phillips starting in center.
The 30-year-old Kiermaier is 1 for 11 this season after batting .217 with three homers in 49 games for the AL champions in 2020.
Right-hander Chris Mazza was optioned to the alternate site.