Piscotty had been out since June 30 with a sprained right knee. He played five games in the minors while rehabbing and batted .304 with one home run and three RBIs.
To make room on the roster, the A's optioned second baseman Franklin Barreto to Triple-A Las Vegas on Saturday. Additionally, right-hander Ryan Dull was designated for assignment and pitcher Andrew Triggs was released.
Garneau batted .232 with two home runs and seven RBIs in 28 games this season with the Angels.