Johnson (elbow) began a rehab assignment with the Orioles' Florida Complex League affiliate, Rich Dubroff of BaltimoreBaseball.com reports.
Johnson was acquired last August as part of a three-team trade and subsequently underwent Tommy John surgery. He hasn't been ruled out for the year, but the right-hander will likely need multiple weeks of rehab appearances before settling in at either High-A Aberdeen or Double-A Bowie to close out the campaign.