Texas coach Mack Brown announced the expected switch Monday, two days after McCoy and Ash came off the bench in the second quarter and rotated snaps while leading the Longhorns to a come-from-behind 17-16 win over BYU.
Ash, a freshman who is still primarily a runner, shook up BYU's defense by directing an option attack that took advantage of Texas' speed.
McCoy, a sophomore whose older brother is former Texas quarterback Colt McCoy, at this point is the better passer and took most of the snaps on the drive to the winning touchdown in the fourth quarter.
Texas (2-0) also announced another big change on offense, with freshman Malcolm Brown listed as the starting tailback ahead of senior Fozzy Whittaker. Brown leads Texas in rushing with 154 yards on 30 carries.
"The one thing we've committed to ourselves and our team is that if something's not working, change it," Brown said. "It's a product of are you moving the ball? Are you scoring points? As a staff, we said let's try something else. The combination of David and Case worked."
While McCoy is likely to get the first snaps against UCLA, Brown said the game plan will dictate how much the quarterbacks rotate against the Bruins. McCoy and Ash combined to complete 8 of 11 passes for 92 yards against BYU, and Ash also ran for 39 yards.
Gilbert won a tense training camp battle to keep his job and seemed settled in the position after directing a stable if unspectacular season-opening 34-9 win against Rice.
But Gilbert was just 2 of 8 passing for 8 yards and two interceptions against BYU and was loudly booed by the home crowd. Down 13-0 and sensing the game was slipping away, Brown and offensive co-coordinator Bryan Harsin pulled Gilbert off the field.
"I'll give Garrett a lot of credit because he was the biggest cheerleader on the sideline. He wants Texas to win," Brown said.
The change culminates a long and steady fall for the former national high school player of the year from nearby Lake Travis. Gilbert was thrust into action as a freshman in the 2010 BCS championship game when Colt McCoy was hurt early and nearly rallied Texas to a win. He was expected to easily slide into the role of Texas' next great quarterback..
Instead, Gilbert struggled badly to live up to those expectations and shouldered much of the blame last year from Texas fans for the Longhorns' first losing season since 1997. Gilbert had a penchant for turnovers -- 19 interceptions against 12 touchdown passes in 14 games as a starter -- and his quiet personality never seemed at ease in the role of leader of the offense.
Case McCoy's teammates said his leadership and command of the huddle were immediately evident against BYU.
"I need ya'll guys to trust me," McCoy told his teammates, according to freshman wide receiver Jaxon Shipley, McCoy's roommate.
"He's real emotional as far as leadership goes," Shipley said.
Case McCoy was not made available to reporters on Monday, but said after the game he went in thinking he had to take charge of an offense that was getting angry and frustrated.
"That's one thing I had to tell them was to calm down," he said. "We have been playing this game since we were in the seventh grade. Who cares if there is 100,000 people out there? Just play your game and play hard."
Ash said after the game the team pulled together behind the young quarterbacks.
"Adversity can do two things: split you apart or pull you together. We pulled together tonight and pulled out a win," Ash said.