Lacy suffered the injury at the end of his first carry of the game, a 10-yard run on the Packers' opening series.
Lacy took a hit from Redskins safety Brandon Meriweather, who was not penalized on the play. The Packers took Lacy to the locker room and ruled him out for the remainder of the game.
"I don't know what the outcome was," Packers coach Mike McCarthy said. "Hopefully he's OK."
"At first I was amped up, but after I saw that he was down, you don't wish that on nobody's life, so I kind of bowed my head and prayed to God that he was OK," Starks, who finished with 132 yards and a touchdown on 20 carries, said of Lacy.
Meriweather also was knocked out of the game with a concussion in the second quarter, when he again led with his helmet on a hit against Starks. Meriweather laid on the ground for several minutes before walking off to the locker room.
It was Meriweather's first regular-season game since suffering a knee injury on Nov. 18. He has yet to play a full game since joining the Redskins prior to last season.
ESPN.com's John Keim contributed to this report.