Start Chris Johnson or David Johnson. Andre Ellington is out and I lean to David Johnson as he is a rookie with decent production. Who gets more points this weekend?
Short term (next week or two) you want to play Chris. If you can afford the bench spot, I think David is a good stash. Could be a significant contributor by week 6 or 7, if not then after their bye starting week 10. Given the Cards next 5 matchups (SF, STL, DET, PIT, BAL) I'm not sure CJ is worth rostering because of the match-ups (PIT being the only good one) unless you sorely need a RB to start.
Chris Johnson is going to get bulk of the work this week, BUT in the long run David Johnson will get more and hopefully be the GUY! I have him also and he has major upside if he can get in there consistently. Not sure what your options are, but for this week I read they are going to go with Chris more. David's projection started at 9.1 and dropped to 8.1. I REALLY like David Johnson though and I know Arians is big on him too! We just need Chris to stink it up!