SANTA CLARA, Calif. – San Francisco 49ers star running back Christian McCaffrey will not require surgery on his injured right knee but will miss the remainder of the regular season.

Coach Kyle Shanahan said Monday that McCaffrey will be out at least six weeks after injuring his posterior cruciate ligament in Sunday night’s loss to the Buffalo Bills. McCaffrey will be placed on injured reserve for the second time this season after missing the first eight games with Achilles tendonitis.

“I just feel for him,” Shanahan said. “It’s been a really frustrating year for him. He worked his ass off to get back to this point and I think he felt really good and was close to hitting the ground running and just had this really unfortunate injury last night. I know he’s as devastated as everyone else, but he’ll get through this. He’s a great player, a great person and an incredible 49er. He will be back stronger than ever to help us next year.”

The 49ers also lost McCaffrey’s backup to injury, and Jordan Mason will also go on injured reserve after suffering a high ankle sprain in the game against the Bills.

San Francisco (5-7) sits in last place in the NFC West standings, two wins behind league leader Seattle and is in danger of missing the playoffs a year after losing to Kansas City in the Super Bowl.

It was a frustrating season for McCaffrey, who was named AP Offensive Player of the Year in 2023 and then signed a lucrative contract extension in the offseason.

McCaffrey injured his Achilles tendon early in training camp and missed the first eight games of the season. He rushed for 149 yards on 43 carries in his first three games and struggled to get back to form.

He looked much better early Sunday night, with seven carries for 53 yards, including a 19-yarder that was his longest run of the season. He appeared to injure his knee on an 18-yard run in the second quarter. On his next run, he immediately went down as he swung left on a snow-covered field for a 5-yard loss.

Mason had played well as McCaffrey’s replacement, running for 789 yards and averaging 5.2 yards per carry this season.

San Francisco will turn to rookie Isaac Guerendo and promote Patrick Taylor from the practice squad. Shanahan said the team will also look to add another running back to the mix.

In other injury news from the game, defensive lineman Kevin Givens suffered a torn pectoral muscle and will miss the remainder of the season.

Shanahan said cornerback Deommodore Lenoir should be back at practice Wednesday after missing the game with a knee injury, and that stars Nick Bosa (hip, oblique) and Trent Williams (ankle) will be evaluated later in the week.

Bosa and Williams have missed the last two games. Williams also had to deal with a family tragedy when his wife announced on Instagram that she had given birth to stillborn Trenton O’Brien Williams Jr. on November 24. Sondra Williams also wrote that she was initially pregnant with twins and had already lost the other child earlier in the pregnancy.

Shanahan said Williams spent time with his family last week but is trying to get back to playing.

“He was at the hospital with her and got to meet him and say goodbye,” Shanahan said. “Then he had to cremate him on Friday. He has dealt with it and is working on it. We’re all just trying to be there for him.”