The Pirates placed Johnson on the minor-league 7-day injured list Tuesday due to a left knee injury, Danny Demilio of PittsburghBaseballNow.com reports.
Johnson left Saturday's game for Triple-A Indianapolis after four innings due to the knee issue. He appeared to have suffered the injury while he stumbled over the first-base bag while trying to leg out an infield single. The 22-year-old had to be carted off the field with an air cast, but exact details of the injury have not been revealed.