Look at the Bills roster, minus Nate, and tell me that RG or LG is a bigger need than CB right now. I'll agree DT is, but the G spots on O (while still a need) aren't a bigger hole than CB without Nate. The Bills have no one on the roster who can step into a starting CB role right now. Of course, that could change after FA begins, but as of right now CB would be a major need.
Without Nate I'd say the Bills biggest needs, in order are:
DT
CB
G
WR
LB
TE (though I'm starting to like Royal more and more)
With the improved play of the OL one could actually argue that WR is more of a need than G.