KANSAS CITY, Mo. — Kyle Shanahan was watching as Jimmy Garoppolo scrambled toward the 49ers sideline and, rather than safely duck out of bounds, stepped back to his right and took a wicked shot from Steven Nelson.

At first, Shanahan thought his quarterback had sustained a concussion.

Turns out it may have been a season-ending knee injury.

Garoppolo’s left knee buckled just before the Chiefs’ cornerback delivered a shoulder-to-shoulder blow, and that was the reason he went down so hard. Garoppolo was on the turf for a couple of minutes before he was helped to his feet and eventually carted off late in a 38-27 loss to Kansas City.

“We fear an ACL,” Shanahan said afterward. “We’ll find out more (Monday).”