There was so much promise and positivity after the Packers won their exhibition opener against the New England Patriots. There were no significant injuries in that game, and the team looked sharp in just about every aspect.

That was not the case Sunday for Green Bay. A lot went wrong, including something that could seriously derail the team’s entire season.

Here are the five biggest takeaways from the Packers’ 24-19 loss to the Pittsburgh Steelers:

1. Jordy Nelson’s last play of the year?

It was the dreaded non-contact knee injury. Nelson caught an eight-yard pass on Green Bay’s opening drive, planted to turn upfield and fell to the ground. Nelson hobbled off the field under his own power, and it seemed somewhat encouraging that he was able to walk somewhat normally into the locker room.

It’s not been confirmed by the Packers as of this article’s posting, but multiple reports surfaced that Nelson tore his ACL. That would be a season-ending injury for Nelson, and one that would significantly hurt Green Bay’s Super Bowl chances.

Over the past two seasons, Nelson has 183 receptions for 2,833 yards and 21 touchdowns. He was a second-team All-Pro and a Pro Bowl selection in 2014, while being rated as the second-best overall wide receiver in the NFL by ProFootballFocus. That’s the type of production that can’t just be replaced by the next man up.

2. Other injuries

The most important part of any exhibition game (at least for starting-caliber players) is to not get injured. Obviously, the Nelson situation puts Aug. 23, 2015 down as a terrible day in Packers history. His injury was the most severe, but it wasn’t the only one.

Aaron Rodgers was sacked and hit hard in the end zone for a safety to conclude Green Bay’s second drive. The two points given up on the play don’t matter, but the impact and fall left the league’s reigning MVP with ice wrapped around his right wrist area. That was Rodgers’ final play of the game. He likely would’ve stayed in had it been the regular season, but it’s not a hit that Mike McCarthy ever wants to see his star quarterback take.

As if that’s not enough for one day of "meaningless" (to use Rodgers’ term) football, T.J. Lang and Scott Tolzien were both evaluated for concussions.

3. Don Barclay’s rough preseason

Barclay struggled in the preseason opener as the backup right tackle. With David Bakhtiari’s knee injury, Barclay not only moved to the starting lineup but also became Rodgers’ blind-side protector. However, the individual results from Barclay remained the same.

Barclay allowed the sack on Rodgers for a safety, as well as an additional sack later when Tolzien was in. Barclay was also called for holding on a second-and-1 play, which turned a great down-and-distance situation into a second-and-long.

The Packers have been supportive of Barclay as he tries to regain his form after a season-ending torn ACL last year. Thus far, however, Barclay doesn’t look ready.

4. Special teams issues

It was a rough day for Green Bay’s special teams. The Packers are hoping for different results this season under new coordinator Ron Zook, but what they got Sunday looked a lot like Shawn Slocum’s group from 2014.

There were three special teams penalties and one almost very costly mistake. Myles White had a 15-yard penalty on the punt coverage unit, Adrian Hubbard was called for an illegal block on a punt return and James Vaughters was flagged for roughing the punter – which gave Pittsburgh a new set of downs.

Jeff Janis let a punt return go right through his arms. Literally. Or at least that’s what the referees decided. Another inch to the left or right and the ball would have surely hit Janis and been ruled a fumble. The Steelers recovered Janis’ muffed catch attempt right near the goal line and would have been back on offense just a couple yards away from a touchdown. That play will likely end any hope Janis had of contributing as a punt returner in the regular season.

Tim Masthay was rated by ProFootballFocus as the worst punter in the NFL last season. That prompted Green Bay to sign Cody Mandell as training-camp competition for Masthay, but that only lasted a couple weeks before Mandell was released. Well, on Sunday, Masthay did little to make the Packers feel good about their punting situation. Masthay had a 29-yard punt, a 31-yarder and a 36-yard punt. That is nowhere near good enough.

5. Some young players sink, others swim

Let’s start with the positive, and no rookie is more deserving of praise than undrafted cornerback LaDarius Gunter. The hype surrounding him has grown to a point where it’d seem logical for Gunter to eventually fall short of expectations, but he exceeded them once again in this game. He had two goal-line pass breakups in one-on-one coverage, including one from Ben Roethlisberger to Martavis Bryant. Gunter also ripped a short pass away from a receiver that would have otherwise been a completed catch.

In his first preseason action, first-round pick Damarious Randall came up with an interception on a 50-50 ball. Second-year defensive lineman Bruce Gaston continued making a strong push for a roster spot with a sack. Aaron Ripkowski made two more quality solo tackles on special teams. Richard Rodgers looks poised for a possible breakout season with the way he’s catching the ball and making plays.

On the down side, running back Alonzo Harris didn’t do himself any favors by dropping a screen pass on a play in which he would have had a ton of room to run. It only took two plays into the game for Ha Ha Clinton-Dix to miss a tackle on a running back. In New England last week, Clinton-Dix missed three tackles in just 13 snaps.