To me this game isn't easier or harder with or without him. David Ash is not good enough to be THAT difference or a factor in the game.
Texas beat OU last year in their rivalry game on the strength of an INT return for a TD and long kick return for a TD, that along with a dogged persistence to rush the football in hopes that the D would play well enough to let McCoy (the backup QB) manage the game. OU who typically has a high octane offense managed only 263 Yards of Total Offense.
The Texas 3rd Down Efficiency that game was huge 13/20. Texas only avg 4.3 Yards per Rush, but consistently pounded the rock with 60 rushes.
This is the strategy I expect to see them employ vs. BYU. Play 7 to 8 in the box and use their secondary to play physical bump n run with our receivers and hope their athleticism is enough to stifle Hill and force BYU into poor 3rd down conversion percentage and quick 3 and outs.