Well, that was unexpected. According to both Jason Wilde and the Green Bay Press-Gazzette, via Pro Football Talk, Bigby sustained a "serious" knee injury that will keep him sidelined for at least a month.
So, Bigby's out, Aaron Rouse is in, and Anthony Smith is no longer with the team. Derrick Martin is now the team's only proven backup at safety, unless McCarthy wants to slide Charles Woodson to safety, which has proven to be a bad idea. (But what about Jarrett Bush? What about Jarrett Bush?)
How this will affect the defensive playcalling is yet to be seen. Bigby is a big hitter, but is erratic in most other functions of the safety position. Rouse has been called a "capable backup," but I'm not sure I agree. In any case, we saw how conservative Dom Capers became when Nick Collins wasn't available. Hopefully, Bigby's absence won't have the same effect.