Just as the San Francisco 49ers confirmed Christian McCaffrey suffered a season-ending PCL injury in the loss to the Buffalo Bills on Sunday night, his backup is also heading to injured reserve. 

Jordan Mason, who filled in admirably for McCaffrey as he dealt with Achilles tendinitis in both his ankles at the start of the season, will miss at least the next four games due to a high-ankle sprain also suffered in the blowout loss to the Bills. 

The 49ers have no choice but to turn to rookie Isaac Guerendo to be their lead running back for what could possibly be the rest of the season with just five games remaining.

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For McCaffrey, 49ers head coach Kyle Shanahan noted that his star could "potentially" be out for the remainder of the season. 

"I think PCL is usually a couple weeks, but I’m not exactly sure yet," Shanahan said prior to learning that it’s six weeks for him to heal with not that many games remaining on the schedule. 

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At 5-7 after losing three straight, the 49ers are grasping at straws hoping they can go on a run to end the year and get back into the playoffs.

But they currently sit in last place in the NFC West, though the division-leading Seattle Seahawks are only 7-5.

Considering that a running game is crucial for Shanahan’s offense to work, he will be asking a lot of his rookie running back to carry a workload that San Fran was hoping McCaffrey could do after healing his Achilles injuries. 

McCaffrey had 53 rushing yards on seven attempts and two catches for 14 yards before leaving the game on his own power. During one of those runs, he slid after feeling something wasn’t right. 

McCaffrey has played in just four games this season, rushing for 202 yards with no touchdowns on the ground nor through the air. 

As for Mason, he broke out earlier this season, rushing for more than 100 yards in three of his first four games as the lead back with McCaffrey hurt. He slowed down a bit but has still smashed his career highs with 789 yards and three touchdowns.

Guerendo, a fourth-round pick out of Louisville, has tallied 246 yards and two touchdowns on 42 rush attempts over 12 games thus far. 

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San Francisco 49ers star Christian McCaffrey left Sunday night’s 35-10 loss to the Buffalo Bills with a knee injury, but it appeared to be much worse than that.

McCaffrey is believed to have suffered a posterior cruciate ligament (PCL) injury and could "potentially" see his season come to an end, 49ers head coach Kyle Shanahan said.

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"I think PCL is usually a couple weeks, but I’m not exactly sure yet," he added.

The running back was a focal point of the 49ers’ game plan early against the Bills. As the snow came down on the field at Highmark Stadium, McCaffrey ran the ball seven times and was targeted three times through the air.

He had 53 rushing yards and two catches for 14 yards before he left the field.

It was a crushing blow to a team that has been riddled with injuries on both sides of the ball. The snow made matters much worse for San Francisco. Brock Purdy had only 94 passing yards on 18 attempts.

"He’s one of the best running backs in the league," Purdy said of McCaffrey. "When you lose him like that, I think it takes away just sort of how we do some things in the offense."

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Shanahan said he believed McCaffrey hurt himself after getting tackled after an 18-yard run in the second quarter. He went down a few plays later.

"It looked like he just got shoe-stringed there, he was about to break a house call," Shanahan said. "It was frustrating. He had a great week of practice. I could feel his urgency and stuff. Thought he came out great, looking real good."

McCaffrey has been on the shelf for most of the season. The reigning Offensive Player of the Year missed the first eight games of the season with Achilles tendinitis.

In the four games he has played, he has 202 rushing yards as well as 15 catches for 146 yards. He has not scored this season.

The Associated Press contributed to this report. 

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Josh Allen made every play a quarterback could make and more.

Fresh off his engagement to pop star Hailee Steinfeld, Allen had to deal with the driving lake-effect snow that plagued the Buffalo area over the holiday weekend and into Sunday night against the San Francisco 49ers. 

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Allen definitely had more pep in his step as the Bills took down the 49ers, 35-10. Buffalo also clinched the AFC East division for the fifth straight season.

The superstar quarterback got into the scoring column with a 7-yard pass to wide receiver Mack Hollins before halftime. Buffalo had a 21-3 lead going into the locker room but were far from finished.

Allen was credited for a receiving touchdown in the third quarter after Amari Cooper lateralled the pass from Allen back to the quarter, who then ran in for a score. Allen officially had zero catches but seven receiving yards and a receiving touchdown.

To cap off the night, Allen ran for an 8-yard score in the fourth quarter to put the game completely out of reach. Mitchell Trubisky took over from there.

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Allen became the first NFL quarterback to throw for a touchdown, run for a touchdown and catch a touchdown in a single game since the merger, the team said.

He had 148 passing yards on 13 completions as well has 18 rushing yards.

Bills running backs James Cook and Ray Davis also were able to get on the board. Cook had a 65-yard touchdown run while Davis had a 5-yard touchdown scamper. Cook had 107 rushing yards on the night and Davis had 59.

The 49ers had no answer to the Bills’ offense the entire night. Players didn’t appear to get any type of feel for the snowy conditions as they were slipping and sliding across the surface.

San Francisco’s best hope was to get Christian McCaffrey every possible opportunity. But the running back suffered a knee injury in the first half and never returned.

Brock Purdy was under 100 yards passing. He was 11-of-18 with 94 passing yards. He was sacked twice. Isaac Guerendo scored San Francisco’s lone touchdown.

Buffalo is now on a seven-game winning streak and have 10 wins on the season.

San Francisco fell to 5-7 and it looked like their season was about to slip away.

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