As a rule, I'd rather have the #2 WR than the #2 RB due to the high injury likelihood with RBs. This is particularly true with Foster. Gordon is too much of an questionmark at this point.
Depending on how your and his teams look, I would counter with Demaryius and one of your better bench players for Foster and a good WR2 like K. Benjamin, Sanders, S. Smith, etc. so you don't lose too much at WR.