Hi, I just answered this in another thread, so I'll kind of summarize my response here. The problem with David Johnson is the lack of a solid return on value, but I understand wanting to jump ship. I think what you settle for will depend heavily on the state of your roster, but I don't think you'll have any issues fielding a low ball offer from anyone in your league. I'm personally buying him in several leagues for that reason, and I've been offering fringe WR1/2 value guys (like Larry Fitgerald and Brandin Cooks) to try and get him... to varying success.
I think he'll improve as the season progresses, and I think the Cardinals' offense can't help but to get better, especially if they make a likely QB change sometime soon. So I'm holding him wherever I can.

With your roster specifically, I actually think you're deep enough at RB that you don't need to panic. I think with Howard, McCaffrey, and Breida, you can get by just fine until AZ figures it out.