Hornibook is in the concussion protocol and sources close to the program told the Milwaukee Journal Sentinel that backup Jack Coan is likely to make his first collegiate start for the No. 20 Badgers.
During a radio appearance Friday morning, Wisconsin coach Paul Chryst acknowledged that the coaching staff knew all week that Hornibrook might not be medically cleared to play Saturday.
Hornibrook has started all seven games for the Badgers, averaging 179.1 yards passing while throwing for 11 touchdowns against six interceptions. He is completing 59.6 percent of his passes.
He has dealt with concussion issues in the past, missing the 2016 Big Ten Championship Game against Penn State because of a concussion. He also was limited in the Cotton Bowl against Western Michigan that same year after suffering a concussion in the regular-season finale.
The Badgers were hoping to redshirt Coan, who has not played at all this season but appeared in six games as a freshman last season, when he completed all five pass attempts he threw for 36 yards.
A redshirt is still a possibility for the season due to an NCAA rule that allows a player to appear in as many as four games before losing that year of eligibility.
Wisconsin is coming off a 49-20 victory over Illinois in which Hornibrook was 13 of 22 for 188 yards, three touchdowns and two interceptions.
The Badgers are 5-2 overall and 3-1 in the conference, and Northwestern is 4-3 overall and 4-1 in conference play. They are among four teams atop the Big Ten West with one conference loss apiece.