I was posting about starters, but being comprehensive is useful, so let's do that, too.
Higdon is gone. Evans is in purgatory. Those are the ones that matter. There's no word that Evans's absence is forever, but he'll obviously miss Spring Camp. Then there's O'Maury Samuels, but he left because he couldn't find his spot, which won't affect RB production unless the scholarship roster craters.
In any event, you're right if your thought is that RB is the weakest aspect of the offense. I liked Tru Wilson as reminiscent of Mike Hart last year**, and Harbaugh just reported that Christian Turner has "surged" to be the lead back, which is encouraging given what we saw of Wilson. And then there are the incoming TrFr. Charbonnet will have a lot to say about this, if he lives up to his HS laurels.
So even if it is the weakest part of the offense, that's more about the other positions being quite well off than this one being a pit.
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**(the most decisive of the backs - great, quick decisions, always falls forward, good YPC [5.9] with low standard deviation, but not a burner and wasn't as well suited for the old offense as Higdon)