Nelson was scheduled to have an MRI that will determine the extent of the injury.
Coach Mike McCarthy said earlier Sunday he is "looking for good news."
"I'm not going to get into preliminary tests [and] opinions trying to beat someone to be first out there," McCarthy said after the game. "Like I said, I'm hoping and praying for good news tomorrow. That's the way I look at it based on the information I've been given."
Fullback John Kuhn also was willing to wait for more information on Nelson.
"That's a lot of speculation at this point in time," Kuhn said. "You have to hear something definitive before you start formulating opinions."
Quarterback Aaron Rodgers didn't sound as optimistic. Rodgers was somber during his postgame news conference.
"It's difficult to lose a guy like that in a meaningless game," Rodgers said.
Asked whether he wished there was no preseason, Rodgers said: "I think a lot of players around the league probably do. At least cut it down, maybe, to a couple."
Nelson landed awkwardly, and his left knee appeared to buckle after a short reception on the game's first drive. He limped off the field after the noncontact injury and walked to the locker room under his own power but did not return.
The Westgate Las Vegas SuperBook reacted quickly to Nelson's injury. The Packers had been co-favorites, with the Seattle Seahawks, to win the Super Bowl at 9-2 odds. SuperBook oddsmaker Ed Salmons lengthened Green Bay's odds to 6-1. The Packers' season win total also was adjusted from 11 minus-110 under to 11 minus-150 under.
The NFL Network reported earlier Sunday that the initial diagnosis on Nelson was a torn ACL.
Last season, Nelson led the Packers with a career-high 98 catches for a franchise-record 1,519 yards and 13 touchdowns.
Given how much the Packers use three-receiver sets, it could mean significantly more playing time for second-year pro Jeff Janis, who replaced Nelson on Sunday, and rookie third-round pick Ty Montgomery.
Rodgers left the game after the first quarter and was seen icing his right hand and wrist on the sideline. McCarthy said the quarterback took a shot when he was sacked for a safety on the second possession but added that Rodgers could have continued if it were a regular-season game.
"It was a tough day of football," McCarthy said. "A lot of injuries for both teams."
ESPN's David Purdum and ESPN Stats & Information contributed to this report.