I wouldn't worry about RBs just yet. You will likely have to do something eventually at least for bye weeks. Your best hope if for Bryant and Williams to start strong. That would give you really good trade bait to get a better RB2. I wouldn't worry about doing anything just yet though. Not sure you would get enough value.
12 Team League. We start 2 QBs and get .5 PPR, otherwise standard scoring
QBs: Stafford, Fitzpatrick, Collins
RBs: Charles, Mendenhall, Best, Starks, Carter, Karim
WRs: D. Jackson, Marshall, S. Smith, Green,
TE: Hernandez, Cook