Zimmermann joined the Orioles' player pool Monday, Roch Kubatko of MASNSports.com reports.
The lefty will give the Orioles' pitching staff additional depth, but he's unlikely to be called upon to fill a major role. He struggled upon promotion to Triple-A Norfolk last season, posting a 4.89 ERA and a 1.60 WHIP while striking out just 18.9 percent of opposing batters over seven starts.