Johnson's chance at a positive first impression might be his last to make a meaningful impression with Randle, McFadden and Dunbar returning sooner rather than later.
"I'm just trying to make sure I've got all my plays down, make sure my mind's on the game, be focused and do everything I can to help us to come out on top," Johnson said.
Johnson's good fortune to be in this spot is a result of the poor fortune of others.
He went undrafted out of Stephen F. Austin after setting school records in yards in a season (1,683) and touchdowns (23) and being named the Southland Conference's Player of the Year. Teams considered him a little too slow to be a viable option in the NFL.
But teams are often wrong in their evaluations and players can defy odds.
"I like to get the dirty yards, put my head down, not afraid of contact," Johnson said. "I've always been a bigger kid so contact is something I kind of like."
Johnson didn't officially sign with the Cowboys until he was about to get on a plane for Oxnard, California, although the actual agreement took place a few days earlier. He was heading to lunch with his agent, Corey Williams, when Williams' phone rang. The Cowboys were on the other end.
"I really wasn't paying attention," Johnson said. "I'm a young kid so anytime I can be on my phone, my head's down in my phone. I was probably sitting there on Twitter. I don't know. Instagram. That's when he told me and I was just shocked."
How was lunch?
"Got a big fish plate," Johnson said. "Let's just say I left full. It was good."
In June, Johnson took part in the Cowboys' minicamp on a tryout basis. With Darren McFadden slowed by a hamstring strain and Ryan Williams slowed by an aching knee, the Cowboys just needed numbers at the position.
Williams had knee surgery after minicamp and was released from the roster before camp started, opening the spot for Johnson.
McFadden has opened camp on the physically unable to perform list with a hamstring strain. Dunbar sprained an ankle on Aug. 4 and hasn't practiced since. Randle suffered a strained oblique on Monday and was limited in Tuesday's practice.
Even if he were 100 percent healthy, he likely wasn't going to play much or at all against the Chargers.
So they will turn to Johnson on Thursday against the Chargers. From the looks of it, he will get a lot of work. Fullback Tyler Clutts has received more tailback work in practice the last few days than the only other healthy runner on the roster, Lache Seastrunk.
His run as the Cowboys' starting runner likely won't last long. Randle could be back to practice soon. McFadden is scheduled to make his camp debut on Saturday. Dunbar is also close to returning.
While he might not have the best chance to make the active roster, a solid showing Thursday can help him make the practice squad.
"It's big, you know," Johnson said. "All eyes are going to be on the preseason games to see who can play, who can't play. You get a lot of work in during camp but you just never can fully get the full game experience, so I feel like it'll help out a lot."