Vikings coach Leslie Frazier said Tuesday that Jenkins suffered a knee injury in Sunday's loss at Atlanta and Johnson had a "significant" hamstring injury.
Long snapper Cullen Loeffler, who has a lower back fracture, and safety Husain Abdullah (concussion) were also placed on IR. Abdullah had missed the last two games.
Jenkins, who was signed in July after spending his first seven NFL seasons with the Atlanta Falcons, had 38 catches for 466 yards and three touchdowns in 11 games.
Johnson started three of his 11 games and Loeffler appeared in all 11 games as well. Abdullah started all nine games for the Vikings before he went down.
Among the four players signed to the active roster Tuesday was safety Jarrad Page, who was released by the Philadelphia Eagles two weeks ago. The Vikings also signed wide receiver Stephen Burton, long snapper Matt Katula and safety Andrew Sendejo.
They signed tight end Mickey Shuler to the practice squad.