What makes you think that Shivers doesn't have the ability to make defenders miss that our other backs do? I think you are underestimating him, especially if you are comparing him to Asa. Boobee will continue to be the "go-to" back that Malzahn desires in his offense. That leaves Shivers and Joiner. Shivers is immensely talented, as we saw this season and his role will likely increase next season. I'm not at all concerned about the two-deep.
The question last year was "can Kmart be an every down back". We know now that the answer to that question doesn't really matter - because he isn't going to be an every down back for Gus Malzahn, barring injuries. Shivers passed him, and that just is what it is. I would imagine that he will continue to contribute in the same limited capacity that he did this season.
Joiner certainly has potential, but it's hard to speculate when he barely touched the ball.
As for incoming freshman? Who knows at this point.
So we have:
RB3: Kam Martin OR Harold Joiner OR Incoming Freshman?
It's not an absolute disaster, but injuries could become very problematic for us.