There's no doubt that his loss would hurt them, but they've partially been here before, playing a partial season without Nelson. In 2012, he missed several games because of a hamstring injury, and QB Aaron Rodgers's stats...looked about the same in 2012 as they did in 2014. Here's how they overcame his absences and here's one big difference.Initial diagnosis on Packers WR Jordy Nelson is a torn ACL. (via @Rapsheet) pic.twitter.com/K2aOEaPLNe— NFL (@NFL) August 23, 2015
WR Randall Cobb and WR James Jones. These two took over as the Packers's leading receivers in 2012 without Nelson. Cobb's still in town doing his thing, and while Jones is gone, WR Davante Adams is expected to do more in 2015 and can take over Jones's role. Now the Packers still did get some healthy games from Nelson, WR Greg Jennings, and TE Jermichael Finley, and the rest of the 2015 receiving core (Will Montgomery, Jeff Janis, and Andrew Quarless/Richard Rodgers) isn't at those players' 2012 levels. If Aaron Rodgers can coax two 60 reception, 600 yards seasons out of two of those players, they won't be too far off of their 2012 pace in 2015.
RB Eddie Lacy. While the receiver depth in 2015 isn't as strong as it was in 2012, the Packers didn't have a running game in 2012. That season, RB Alex Green was their leading rusher, and his 2012 performance makes their decision to draft Lacy in 2013 even more obvious in hindsight.
As for the rest of the preseason game, it was disappointing to allow 15 unanswered points to end it and lose (even it was the preseason) and there were a couple of culprits:
QB Brett Hundley. He looked good against the Patriots, and looked like a confused rookie against the Steelers. He'll get a couple more chances in their last preseason games to get some on-field experience before he spends most of the regular season on the inactive portion of the 53-man roster.
LT/RT Don Barclay. After a bad game against the Patriots, he was even worse against the Steelers. He got to play 1st team LT while David Bakhtiari is out with a knee injury and he allowed multiple sacks, including the safety while Rodgers was still in at QB and the hit on Scott Tolzien that knocked him out of the game with concussion symptoms. The bigger problem is that they don't have an obvious second choice to replace Barclay. OL JC Tretter played some tackle (poorly) last season, though I haven't seen him lined up at tackle once this preseason, and he's probably the next best in-house option.
Backup and rookie cornerbacks. While CB LaDarius Gunter continues to look like another undrafted CB gem from the U (following in Sam Shields's footsteps) and rookie 1st round CB Damarious Randall had an INT, rookie 2nd round CB Quinten Rollins struggled and so did a number of backups. With the exception of Rollins, none of these DBs should have been expected to make anything other than the practice squad, so it's not exactly the biggest concern.
Penalties. Especially the roughing the passer and punter 4th quarter penalties that helped the Steelers come back. Again, these penalties were committed by players that weren't expected to make the team, so it's not a big problem.