Johnson replaced safety Usama Young on the roster Wednesday as the Browns look for protection at quarterback, where starter Brandon Weeden went out with a sprained right shoulder in Cleveland's 34-12 loss in Denver on Sunday.
Young sustained a thumb injury that likely will require surgery. He joins a battered defensive backfield that is already missing T.J. Ward with a knee injury. Cornerback Sheldon Brown sustained a concussion in Denver. The club also signed defensive back Jordan Mabin to the practice squad.
Johnson played in 26 games for Tampa Bay after being a fifth-round choice in 2008. He was in San Francisco's camp this summer.