It completely sucks to lose a player of Jordy Nelson's caliber for the entire season, and there's no getting around that. He is, in my opinion, one of the five best receivers in the league and is a huge weapon as the focal point of the Packers' passing attack.
With all of that said, if you have to lose a player and you have depth like the Packers do, you would want it to be a wide receiver, because wide receivers are just about the easiest things to replace in all of sports. Seriously. To understand why, we have to look at how replacements actually work. For example, in Major League Baseball, there is basically nothing worse than losing your best starting pitcher, because you don't just get to replace him with your second best starting pitcher. Or your third. Or your fourth. Or your fifth. If your staff ace gets hurt you have to go all the way down the depth chart to your sixth best starting pitcher. This is generally a huge drop-off.
It hurts to lose a shortstop or second baseman too, especially if that player is a star, but at least you get to go to a direct backup, and often you can shuffle existing positions to get your best backup player into the game. It's bad, but it's not like losing a starting pitcher.
In football losing an offensive lineman, especially a left tackle, is pretty bad. Not everyone on the line is fungible, left tackles are hard to come by, and the resulting positional shifts can result in multiple players playing outside of their comfort zones, weakening the unit as a whole. Similarly, losing defensive players can also have a cascade effect with good players moving off-position or inferior players assuming huge roles. The reason losing offensive linemen or defensive players is so bad is that, in the case of the offensive line, the defense dictates how it is attacked, and in the case of the defense, the opposing offense dictates how it is attacked. In such scenarios, weak links become focal points, and replacement players become targets.
Oh, and then there's the horror of losing an MVP-winning quarterback for half a season, like the Packers and Aaron Rodgers in 2013. You don't just replace that kind of player under any circumstances, and you certainly can't come close to the same level of production with names like Seneca Wallace or a first-year-in-the-organization Scott Tolzien.
Wide receiver has no such issue. The offense itself dictates who is targeted in the passing game, and it can selectively emphasize - or de-emphasize - any individual role. Moreover, Jordy Nelson's direct replacement will not see anything close to his 151 targets from 2014. Most of those will simply be apportioned to the existing excellent receiving corps. Randall Cobb is roughly as productive as Nelson, and simply absorbing a sixth of Nelson's targets in addition to his normal allotment would get him roughly to Nelson's 2014 target numbers. For everyone who thinks Davante Adams is a huge breakout candidate, he had 85 fewer targets than Nelson last year and everyone already expected a huge jump for him. Ty Montgomery and future hall of famer Jeff Janis will probably see bigger roles, but again, that was already expected to some extent. And Eddie Lacy, and Richard Rodgers, and James Starks and eventually Andrew Quarless (and who knows, maybe even Jared Abbrederis if he can get back on the field) can all help to pick up the slack.
And Aaron Rodgers gets to decide who should and should not be replacing Jordy Nelson on a play-to-play basis. Nowhere else in sports is it so easy to simply steer a workload to its most efficient participants. Granted, there is a limit to this principle. If you lack depth at receiver like so many Calvin Johnson-based Lions teams before Golden Tate, an injury to Megatron is devastating. Likewise the vintage Andre Johnson Houston Texans with Kevin Walter and Jacoby Jones waiting in the wings. But for a team like the Packers, losing even a great player like Nelson just isn't that big a deal.
This is also why signing a veteran free agent receiver would likely be a mistake. Bringing in a fairly washed up Reggie Wayne wouldn't so much replace Nelson as it would replace a chunk of Randall Cobb, and a chunk of Davante Adams, and Ty Montgomery, and Eddie Lacy, and Richard Rodgers, and Jeff Janis, and...stop me when I get to a player who's worse than 2015 Reggie Wayne.
So yes, it's bad, and this should not be read as saying that Nelson isn't a star, because he is. There will most certainly be a negative effect to losing him, especially if some of the younger guys do not develop into credible deep threats, but in general the Packers are extremely well-prepared to weather the storm and maintain a superb offense, thanks to the depth they have developed on offense. Don't Panic.