The San Francisco 49ers lost more than a game on Sunday at Kansas City.
Garoppolo was hurt late in the fourth quarter while running along the left sideline on a third-and-goal play from the Kansas City 20-yard line before heading out of bounds after a 13-yard run. He was hit by Chiefs cornerback Steven Nelson and his left knee buckled.
After leaving the field and spending several minutes in the medical tent, he was taken by cart to the team's locker room.
"With Jimmy, it's a knee," 49ers head coach Kyle Shanahan said at his postgame press conference. "We fear an ACL."
More tests are scheduled for Monday to determine the extent of Garoppolo's injury.
"Any time you lose your starting quarterback, that is a big deal," Shanahan said. "It was when we lost our starting running back [Jerick McKinnon] too. So, obviously, I feel for him personally. I know how disappointed he is. I haven't got a chance to see him yet. I feel for him."
Garoppolo had passed for 251 yards and two touchdowns. At the time he was injured, he was trying to trim the 49ers' 14-point deficit to seven.
"Jimmy was just trying to make a play," 49ers left tackle Joe Staley said. "We were trying to make a comeback."
Backup quarterback C.J. Beathard, who started five games last season, entered after Garoppolo left and threw an apparent 7-yard touchdown pass to tight end George Kittle on the first play, but it was nullified by a penalty.
The 49ers settled for a field goal but never got the ball back as the Chiefs ran out the clock with three first downs.
With Garoppolo apparently finished for the season, the 49ers (1-2) plan to go with Beathard as the starting quarterback.
"It sucks," Kittle said of Garoppolo's injury. "Jimmy is a great quarterback, the leader of our team. But it's football, and it's next man in. That's just the nature of the game.
"Luckily, we have a guy who has been in the system, and he's had success in C.J. So we're just going to look ahead and try to keep moving forward."
The 49ers are expected to sign another quarterback to the active roster or practice squad while handing the keys to the offense to Beathard.
"I have got a lot of confidence in C.J.," Shanahan said. "He came in today, made a helluva throw on that fourth down. No hesitation. C.J. is a gamer. Everyone in here has a ton of respect for C.J., how he handles himself. He's a man out there, and he's a very good quarterback, and we're fortunate to have him."