Chad Reineke, who allowed four runs over five innings in place of Dallas Braden (inflamed left ankle) on Wednesday, was optioned to Triple-A Sacramento.
Right-hander Jeff Gray has been recalled to take Reineke's spot on the active roster.
"We're trying to bolster the bullpen a little bit," manager Bob Geren said. "We still have another seven days before we get a day off.
"[Gray] has pitched well in late-game situations. We're happy to get him here."
Gray joins the A's for the fourth time this season. He's pitched in three games, throwing 1 2/3 scoreless innings. The 27-year-old was 2-2 with a 1.54 ERA and 16 saves in 37 appearances with Sacramento.
Gray's recall leaves Giambi in limbo. Giambi said on Friday that he's healthy and feels great. He's been eligible to come off the DL since Tuesday.
Geren, who has dodged questions regarding Giambi's status, followed suit Thursday afternoon.
"No, nothing today," Geren said. "I haven't seen him today."
Oakland already has three first basemen on the active roster. Geren wants to play Tommy Everidge every day. Bobby Crosby and Nomar Garciaparra are also available.
To make matters even less clear, another first baseman, Daric Barton, is eligible to come off the DL on Tuesday.
Geren said Barton, who's recovering from a strained left hamstring, is making solid progress and could begin his rehab with Sacramento as soon as Thursday.
Adam Loberstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.