I'd go with Nelson under any circumstances. I'm not sold on Decker yet, but I definitely see Nelson benefitting from Stevie Johnson playing on Sunday. A slowed down Stevie draws McCourty most likely, letting Nelson likely see a lot of balls thrown his way. And the Bills will be throwing a ton.
Re: Decker, I dont think he's a great play even if Lloyd's out. Jordy is a much tougher call. When GB goes to CHI, they tend to run more than usual. Maybe its the wind, but Rodgers tends to tone down the passing game there. However, I consider Jordy to be taking on Driver's role, and Driver usually gets more looks as the possession guy.
I'd stick with Nelson, who has big time fantasy upside this week.