School trainers announced Wednesday that Ash would not travel with the team to Ames, either.
With Ash sidelined, senior Case McCoy will get his second start this season at quarterback. Longhorns coach Mack Brown, however, indicated earlier in the week that, if Ash was out, there would be a scenario in which freshman Tyrone Swoopes could play, too.
Swoopes hasn't played a snap this season and has been redshirting.
"He's very talented, he's got a very strong arm," Brown said of Swoopes. "He's doing some of the things we saw in the spring game. He's running well, making some good plays and some freshman mistakes."
Ash originally suffered the head injury in a Week 2 loss at BYU. McCoy started for him the following game against Ole Miss. Ash returned for Texas' Sept. 21 win over Kansas State, but was ruled out from returning to the field in the second half during halftime.