I think the answer is going to be that everyone will have to step up. I'm looking back at the 2012 season, when Nelson and WR Greg Jennings were lost for a combined 13 games due to injuries, and here's the statistical receiving breakdown from Football Outsiders:
Maybe WR Ty Montgomery will surprise me, or WR Jeff Janis, but I look at what Davante Adams did in 2014 and find it hard to believe either one of them could be better as a rookie than Adams. Cobb excelled as a rookie in 2011 but he's the exception, not the rule. WR Myles White has his moments in the preseason, along with some drops, and seems like an unlikely savior.
Other than WR Koren Robinson in 2007, I can't think of a veteran receiver that the Packers even tried to plug into Mike McCarthy's offense, and Robinson wasn't a success. James Jones is mentioned now as a trade target, if the Giants do release or trade him, but the Packers drafted three receivers in 2014 to replace him. Plus the Giants wouldn't gain much in trade and Jones has gotten off to a strong start with them. I can't see any other veteran free agent fit, and there should be some demand when teams like the Bears and Panthers are also trying to replace injured wide receivers.
So it's a tough situation for Green Bay. Instead of relying on Nelson to get open against whatever tight coverage faced him, the Packers are probably going to have to get creative with different personnel and spreading the ball around to many receivers.