"It's been a couple days it's been bothering him some, so we've been holding him back the last few to be proactive," said manager Bob Melvin. "I don't think there's any problems there."
Melvin noted that the 30-year-old Gentry, acquired by the A's with Josh Lindblom from Texas this winter, does not have a history of back issues.
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.