Williams injured his ankle during Thursday's preseason finale against the Chargers, Eric Branch of The San Francisco Chronicle reports.
Prior to his early exit, Williams gained 53 yards on 10 carries but also lost a fumble. He struggled with ball security in college and again during training camp, perhaps thwarting his own bid for a significant role in the San Francisco backfield. With Tim Hightower also failing to impress this preseason, undrafted rookie Matt Breida has a shot to open the season as Carlos Hyde's top backup.