According to ESPN's Adam Schefter, Lacy will have ankle surgery and be placed on injured reserve. NFL Network's Ian Rapoport reported Lacy got a second opinion that confirmed his ankle is more than sprained and needs surgery.
Rapoport added the Packers will make a determination later about a potential return for Lacy this season and that Don Jackson will be Green Bay's primary running back on Thursday night against the Chicago Bears.
Rapoport noted the Packers will also have Ty Montgomery in the backfield Thursday night alongside Jackson against the Bears. He added it will be difficult for Chicago defensive coordinator Vic Fangio to scout Jackson, who has had no game snaps, even in the preseason.
Per ESPN.com's Field Yates, with Lacy headed to injured reserve, he won't be eligible to return until Week 15 even if he can come back.
Schefter noted Lacy will be an unrestricted free agent after this season, so getting his ankle back to 100 percent before March should be of the utmost importance as he seeks a new deal during the offseason.
Lacy had ankle problems during the preseason, leaving a practice in August, but there had been no signs of lingering effects once the regular season began.
Lacy was enjoying an excellent bounce-back following his disastrous 2015 campaign. The former Pro Bowler averaged a career-high 5.1 yards per carry in five games.
The Packers are scrambling to find answers for an offense that ranks 17th in points per game (22.8) and 25th in yards per game (331.8). Lacy's absence will only make their search more difficult and put more pressure on quarterback Aaron Rodgers.