San Francisco 49ers star running back Christian McCaffrey’s season is in jeopardy after he sustained what is believed to be a posterior cruciate ligament injury in Sunday’s 35-10 loss to the Buffalo Bills in Orchard Park, N.Y.

“I think potentially (it’s season-ending),” 49ers head coach Kyle Shanahan said after the game. “I think PCL is usually a couple weeks, but I’m not exactly sure yet.”

McCaffrey will undergo additional testing on Monday to determine the severity of the injury.

He appeared to sustain the injury after being tripped by Bills safety Taylor Rapp at the conclusion of an 18-yard gain with 12:58 remaining in the second quarter on Sunday. McCaffrey exited the game briefly before returning to take a pitch to the right, only to elect to slide to the field without contact for a 5-yard loss.

The 49ers originally listed McCaffrey as questionable to return shortly after he left the playing surface for further examination. However, Shanahan later told NBC that the 28-year-old was done for the game.

“It was frustrating,” Shanahan said after the game. “He had a great week of practice and I could feel his urgency and stuff and thought he came out great, looking really good, and it looked like he just got his shoestring there. … I hurt for him, and tough for our team not having him.”

The 49ers (5-7) have lost three in a row heading into next Sunday’s game against the Chicago Bears (4-8) in Santa Clara, Calif. San Francisco resides two games behind the NFC West-leading Seattle Seahawks (7-5) with five games remaining on the schedule.

Seattle and San Francisco split their season series.

The 49ers once again are expected to lean on Jordan Mason, who leads the team this season in carries (153) and rushing yards (789) to go along with three touchdowns.

McCaffrey had 53 yards on seven carries on Sunday night and caught two passes for 14 yards before exiting. He was playing in just his fourth game of the season after missing the first eight due to Achilles tendinitis in both legs.

McCaffrey was the NFL Offensive Player of the Year last season, when he led the league with 2,023 yards from scrimmage: a league-leading 1,459 rushing yards and 14 touchdowns plus 67 catches for 564 yards and seven scores.

McCaffrey hasn’t scored a touchdown in his four appearances this season. He has rushed for 202 yards on 50 carries and caught 15 passes for 146 yards.