As a Bears fan, it pains me to say p/u Nelson and drop Knox. GB plays a lot 3wr sets and Nelson gets a bunch of targets. If you need a rb, it depends on your league. Ppr: Bush. Non-ppr: Starks. Knox is a nice player, but his stats are a dime-a-dozen in FF. Also, don't even think about dropping Colston.
I would drop Moreno and pick up either Starks or Sproles, whichever one rings your bell. If your league gives points for return yards it would be Sproles, hands down.
I would drop Knox and go with Burress or Nelson, whichever teeters your totter. Nelson's plus - Jennings handcuff. Burress plus - if he can keep from shooting himself again he is is going to be a beast. In no time I think he will be Sanchez's safety blanket and clear target for jump balls in the red zone.