Just my opinions:
Option 1: Call Johnson's agent and get in on the action and see what his asking price is. Unfortunately, because the Cowboys waited, he now looks to have a market and that will cost more.
Option 2: Bring another vet back into camp. They worked out Ben Tate a few months ago, if he's still in shape, bring him to camp and at least have another option on the roster.
Option 3: Call around and inquire about trading for a young back with some chance to be a starter. Montee Ball is my first choice, but I would call the Jets about Zac Stacy too.
What I would not do is sit on my hands waiting for the next injury to happen to one of these guys. These injuries are minor, but they should be a wake-up call. They aren't devastating now, but during the season they could be.