Garciaparra, out with a strained right calf since April 29, will be used in the same manner in which he opened the season.
"It's day to day," Geren said of his availability. "We'll monitor him and check in with him every day. He won't be playing every day but we never expected him to do that."
Garciaparra was hitting .231 with a home run and seven RBI in 12 games before going on the disabled list. He's a career .330 hitter in Interleague Play with 11 triples, tied with Tampa Bay's Carl Crawford for the second most in Interleague Play.
Right-hander Chris Schroder, who was called up from Triple-A on Tuesday but has not pitched, was sent back to Sacramento to make room for Garciaparra.
"When we added Garciaparra, we had to balance the roster," Geren said. "I felt a pitcher had to be sent down."
Oakland is back to a 12-man pitching staff following the move.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.