The reality is that Johnson was never great at making defenders miss in small spaces, and he doesn't fight through tackles between the numbers. He's a boom or bust runner that combines a lot of one-yard gains with the occasional big play. The Cardinals already have a big play threat in Ellington, and really need a chain-mover that could help on third downs. Johnson was workmanlike as a member of the Jets last year, where he got 179 touches.
With final cutdowns only a few weeks away, Johnson will have to impress coach Bruce Arians quickly to make the team. Johnson is certainly not going to help on special teams, so he has to make the case that he can share the load with Ellington.