The Longhorns starting quarterback suffered head and shoulder injuries late in last week's loss against BYU.
Senior Case McCoy will likely start.
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Ash is 39 for 62 passing this season for 594 yards, six touchdowns and two interceptions. He's also rushed for a team-best 125 yards and another score.
McCoy is 7 of 12 passing for 75 yards. He threw for 722 yards and six TDs last season and for 1,034 yards and seven scores as a sophomore.
