Johnson left practice early Saturday due to an apparent right knee injury, Matt Parrino of NYup.com reports.
Johnson was participating in 1-on-1 drills when he went down and immediately grabbed his right knee. The running back was escorted to the sideline, and then to the locker room, by team trainers. There's no word yet on the exact nature or extent of Johnson's injury, but any absence lingering into the regular season would be a blow to Buffalo's backfield depth, as the versatile tailback is expected to again work in a dual-threat role behind James Cook and Ray Davis.