Sebastian had 11 tackles, one-half tackle for loss and one interception before going down late in the second quarter against Northwestern.
"He was the heart and soul of our defense," linebacker Jalen Jefferson said of the hard-hitting Sebastian.
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Sebastian had previously suffered a neck injury during a frightening collision in fall camp, but recovered in time to make his fifth career start.
"He's down, obviously," Dykes told BearTerritory.net. "He's a tough kid, and he's dealt with a lot here over the last month. He's very disappointed."
Sebastian is eligible to redshirt and will have two seasons of eligibility remaining.
Two other Cal starters, middle linebacker Nick Forbes (back) and defensive end Brennan Scarlett (hand), did not play against the Wildcats and their status for this Saturday's game against Portland State is questionable at best.