49ers put running backs Christian McCaffrey, Jordan Mason on injured reserve

Just four weeks after returning to the field, San Francisco 49ers running back Christian McCaffrey is out again.

49ers head coach Kyle Shanahan told reporters Monday that McCaffrey suffered a posterior cruciate ligament (PCL) injury during Sunday's 35-10 loss to the Buffalo Bills, and will be placed on injured reserve as result. McCaffrey's recovery is expected to be six weeks, meaning that the rest of his season is in jeopardy.

Additionally, Jordan Mason — who took over as RB1 after McCaffrey left Sunday night's game — is going on IR after suffering a high ankle sprain.

During the second quarter of Sunday night's game, McCaffrey suddenly pulled up on a run after suffering the non-contact injury. He went into the blue medical tent moments later, and then walked back to the locker room under his own power.

The 49ers initially ruled McCaffrey questionable to return, before ruling him out at the start of the second half. After the game, Shanahan told reporters that McCaffrey's PCL injury was "potentially" season-ending, but did not provide further specifics.

With both McCaffrey and Mason out, rookie Isaac Guerendo is likely to step in as San Francisco's starting running back.

McCaffrey made his season debut at the start of November after missing the first eight games of the season due to Achilles tendinitis and calf tightness. Soon after going on IR ahead of Week 2, McCaffrey flew to Germany to receive treatment from a specialist.

McCaffrey has played in four games since returning, racking up 50 carries and 202 yards, as well as 15 receptions for 146 yards. He was the leading rusher in his first three games back with San Francisco.

Mason, taking on a bigger role this season in McCaffrey's absence, has played in 12 games. The third-year running back had picked up 153 carries for 789 yards and three touchdowns so far this year, prior to going on IR.

The 49ers have fallen to a 5-7 record after Sunday's loss and sit at the bottom of the NFC West. McCaffrey's return last month was a major boon to the team, but San Francisco has a slew of other problems that the reigning Offensive Player of the Year alone couldn't fix. Injuries have also hampered the team's performance, with several key players — including receiver Brandon Aiyuk — on injured reserve.