Reddick sustained the injury during a defensive drills on Friday, but thought he would only miss three or four days.
Jane Lee of MLB.com reported Friday that Reddick wasn't experiencing as much pain as he had when he sustained a "Grade 2 left oblique strain in 2009, which forced him to miss two months." Lee reports Reddick has not had an MRI yet.
"We know for sure he's not doing anything for two weeks, and then it's a progression to get him ready again," said Melvin. "You have to start from zero again. I would think Opening Day would be a stretch, but he has been a quick healer."
Last season with the A's, Reddick hit .264 with 54 RBI and 12 home runs in 109 games.