The 38-year-old first baseman and designated hitter had returned to Oakland this season after seven years with the New York Yankees.
Giambi, the 2000 AL MVP for the A's, has been on the disabled list since July 20 with a strained right quadriceps muscle. His legs have been bothering him for months, limiting his time in the field. The injuries, however, weren't considered career threatening.
A five-time AL All-Star, Giambi was batting just .193 -- lowest in the majors when he went on the DL. He has 11 home runs and 40 RBIs in 83 games this season. When he was placed on the DL, general manager Billy Beane and manager Bob Geren said a break might help him.
A phone message left for Giambi's agent, Arn Tellem, wasn't immediately returned.
Beane re-signed Giambi in hopes he would ignite a dormant offense to a lineup that in 2008 lacked punch. Instead, Giambi had a nine-game stretch in June during which he was 2 for 26 (.077). He hit .152 in June for the lowest average for a month in his 15-year career.
New York declined its $22 million option on Giambi after last season, choosing to pay him a $5 million buyout. That's how Giambi wound up back in Oakland, the team that drafted him in the second round in 1992 out of Long Beach State. He made his big league debut three years later.