Garbers returned to the sideline in the third quarter with his right arm in a sling.
Coach Justin Wilcox did not have an update on Garbers' injury in his postgame news conference. He said he expects to have more details Monday.
Garbers got hurt when he landed on his throwing arm on a scramble late in the second quarter for the 15th-ranked Golden Bears. The redshirt sophomore -- who had completed 9 of 12 passes for a 117 yards and a touchdown -- was helped off the field, giving high-fives to teammates with his left arm, and went directly to the locker room.
The Bears (4-1, 1-1 Pac-12) struggled to generate anything in the passing game after Garbers got hurt and was replaced by Devon Modster.
Modster finished 5-of-14 for 23 yards and threw an interception in the end zone to deny Cal a chance at a field goal at the end of the first half.
"Honestly, I think I played terrible," Modster said. "I could have played a lot better, just missing a lot of receivers."
Garbers was coming off his best game at Cal, throwing for 357 yards and four touchdowns in a victory last week at Mississippi.
The Bears' loss left the Pac-12 with no undefeated teams before the end of September.
The Associated Press contributed to this report.