Coach Bruce Arians doesn't like the word "need", but the Cardinals were lacking a big, physical back. David Johnson will contend for the starting job, and at least should be a nice complement to Andre Ellington. Johnson is big and he ran the 40-yard dash at the combine in 4.5 seconds, which is plenty fast for a man his size.
The Cardinals have been trying to add a bigger back to complement Ellington, and Johnson fits the bill. It’s starting to look like a committee in Arizona, so keep an eye on this camp battle to see if Johnson’s able to unseat Ellington. For now, Ellington should be able to hold onto the job with Johnson serving in a change-of-pace role.