Johnson was removed from Triple-A Indianapolis' game against Iowa on Saturday after four innings due to an apparent left knee injury, Danny Demilio of PittsburghBaseballNow.com reports.
Johnson sustained the injury while tumbling over first base as he attempted to beat out a grounder for an infield single in the bottom of the fourth. Per Jason Mackey of MLB.com, Johnson needed to be carted off the field and was fitted for an air cast. The Pirates haven't provided an updated diagnosis for Johnson, but the knee issue seems likely to force the 22-year-old to Indianapolis' injured list.