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 further tests on his knee and seeking a second opinion, the sources told Schefter. Some within the Lions organization believe Montgomery will miss this season, the sources said.

He is one of several key players injured on 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 broken jaw in the loss, sources told Schefter. Davis will undergo surgery and be sidelined indefinitely, the sources said.

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

The injuries to McNeill, Davis and Dorsey are the latest setbacks for the Lions’ defense, which had 13 defensive players on the injured reserve list before Sunday’s game. This includes star defender Aidan Hutchinson, who suffered a broken 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. In October, he signed a two-year, $18.25 million contract extension with the Lions.

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 contract extension earlier this season and had helped stabilize Detroit’s defensive line following Hutchinson’s injury. He finished the season with 25 tackles and 3.5 sacks.

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

The loss to the Bills ended the Lions’ 11-game winning streak. The Lions (12-2) are still atop the NFC playoffs and hold a tiebreaker over the Philadelphia Eagles (12-2) with three games left based on winning percentage in conference games.

ESPN’s Eric Woodyard contributed to this report.