Peter Aiken / Stringer

KANSAS CITY — The 49ers’ locker room looked gloomier than after the typical loss, as you’d expect, with the same thought seemingly filling everyone’s head: Jimmy Garoppolo’s knee injury.

In the fourth quarter of San Francisco’s 38-27 loss to the Kansas City Chiefs, Garoppolo scrambled toward the left sideline, planted his left foot awkwardly, and was leveled out of bounds. He was later carted off the field. 49ers head coach Kyle Shanahan fears Garoppolo tore his ACL, though the 49ers won’t know for sure — or disclose the information — until at least Monday.

Perhaps the best indicator of the locker room’s postgame temperament was Reuben Foster, who proclaimed Friday that he is naturally “happy-go-jolly.” If anyone had a reason to be excited for Sunday, it was him. It was his first game back after serving a two-game suspension. The always-upbeat Foster flashed a couple classic smiles.

But the 11-point loss, and more importantly, Garoppolo’s injury, were still fresh.

“I’m down right now,” he said. “We have big hopes for Jimmy. He made us who we are.”

So was the entire locker room, as you’d expect, with the franchise quarterback, the $137.5 million man, and the newly-minted face of the organization laid up next door on the training table.

“It’s a hard pill to swallow,” Foster said. “That’s Jimmy G.”

Garoppolo’s emergence last year, famously winning all five of his starts after being traded to close the season, yielded optimism for 2018 and beyond. The 49ers spent the recent offseason building around Garoppolo both in free agency and in the draft. When Richard Sherman signed with the 49ers, he cited Garoppolo as a major factor.

Sunday epitomized the difference between 2018 and recent years. Down 28 points in the first half, playing one of the league’s hottest teams in a raucous environment, the 49ers were never out of the game. They scored 18 straight points to cut the lead to 11 with more than 17 minutes remaining— and momentum on their side.

More importantly, they believed they were never out of it. Garoppolo, at halftime, was the one instilling confidence in his teammates.

“That’s the type of leader he is,” Marquise Goodwin said. “Talk the talk, walk the walk, encouraging words to the whole offense not to give up.”

The way Garoppolo injured his knee embodied that resilience. On third and goal at the Kansas City 20, he scrambled out of the pocket after surveying the field. He gained 11 yards. 49ers offensive tackle Joe Staley commended Garoppolo for putting his head down rather than sliding or running out of bounds.

“Jimmy was just trying to be a ballplayer, trying to make a play, trying to get some extra yards for his team,” Staley said. “It was just one of those things.”

Staley said he initially thought Garoppolo’s injury was a concussion because he was hit hard. Once he was carted off the field, it became clear he had suffered a leg injury.

Assuming that the early fears of a torn ACL are true, Garoppolo’s injury changes the trajectory of the 49ers’ 2018 season. It’s now C.J. Beathard time, just as it was less than a year ago. As a rookie, Beathard inherited the starting job for a winless team and led the 49ers to their first victory of the season in Week 10. Three weeks later, Garoppolo elevated the 49ers to another level, showing that a quarterback upgrade helps everything else fall into place.

Garoppolo’s 2018 campaign went up and down, but he was impressive in the loss. He completed 20 of 30 passing attempts for 251 yards and two touchdowns, while engineering a serious second-half push.

“It’s a downer,” Goodwin said on Garoppolo’s injury. “But in the game we play, it happens sometimes. You hate to see one of your leaders go down. It just means C.J. has to step up on the plate.”

After the game, Garoppolo’s family members waited for about 30 minutes in the hallway outside the visiting locker room to see him. His teammates were less patient. Once Goodwin exited the locker room, he interviewed with reporters for about two minutes. He was asked about Garoppolo’s whereabouts.

“I am looking for him right now, but I can’t find him,” Goodwin said.

Beathard, the player whom Garoppolo’s injury affects most, had better success. He was one of the 49ers players who checked on Garoppolo in the training room.

Their short conversation represents the state of the 49ers moving forward.

“I told him I’d be praying for him, and I love him,” Beathard said. “He told me to lead these guys.”