Detroit Lions running back David Montgomery is out indefinitely after suffering a sprained MCL in Sunday's 48-42 loss to the Buffalo Bills, sources told ESPN's Adam Schefter on Monday.

Montgomery is undergoing additional testing on his knee and getting a second opinion, the sources told Schefter. There are some within the Lions organization who believe Montgomery will be lost for the season, the sources said.

He is one of several key players who were injured Sunday, including several key defensive players. Lions starting defensive tackle Alim McNeill suffered a torn ACL, ending his season, and starting cornerback Carlton Davis III suffered a fractured jaw in the loss, sources told Schefter. Davis will undergo surgery and be sidelined indefinitely, the sources said.

Lions coach Dan Campbell had said Sunday that both McNeill and Davis would likely miss the rest of the season. The Lions also lost cornerback Khalil Dorsey to a season-ending ankle injury in Sunday's game, Campbell said.

The injuries to McNeill, Davis and Dorsey are the latest setbacks to a Lions defense that had 13 defensive players on the injured reserve list entering Sunday's game. That includes star defensive end Aidan Hutchinson, who suffered a fractured tibia and fibula in Week 6.

Montgomery is second on the Lions with 775 rushing yards and leads the team with 12 rushing touchdowns. He also has 36 receptions for 341 yards. He signed a two-year, $18.25 million extension with the Lions in October.

Montgomery is part of the Lions' one-two punch at running back with Jahmyr Gibbs, who leads the team with 1,047 rushing yards and has scored 14 total touchdowns (11 rushing).

McNeill signed a four-year, $97 million extension earlier this season and helped stabilize Detroit's defensive line after Hutchinson's injury. He finishes the season with 25 tackles and 3.5 sacks.

Davis, acquired in an offseason trade with the Tampa Bay Buccaneers, has 2 interceptions, 11 passes defensed and 56 tackles this season. He will be an unrestricted free agent after the season.

The loss to the Bills snapped the Lions' 11-game winning streak. The Lions (12-2) still are in the top spot in the NFC playoff bracket, holding the tiebreaker over the Philadelphia Eagles (12-2) based on winning percentage in conference games, with three games to go.

ESPN's Eric Woodyard contributed to this report.