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Mack Brown was convinced Manny Diaz could turn things around after last year's disaster on defense because a Diaz-led the Big 12's best defense against the run and pass in 2011 and had started to show improvement toward the end of 2012.

But after watching his defense give up a school-record 550 yards rushing on the ground in a 40-21 upset loss at BYU Saturday night - with no improvement after halftime - Mack Brown had seen enough.

In a shocking development that tells you just what a make-or-break season this is for Brown, the intensely loyal Texas coach, who has never fired an assistant mid-season until Sunday, cut ties with Diaz after two games, Diaz will be reassigned to a position within the athletics department. Orangebloods.com was the first to report the news.

Brown is turning to Greg Robinson, who served as Texas defensive coordinator in 2004 and who had been working for UT as a football analyst this season.

Part of Robinson's work as an analyst for Texas has been to scout opponents and make suggestions about possible game plans.

A source close to the situation told Orangebloods.com Robinson's suggested game plan for BYU was to put 8 and 9 defenders in the box, play man coverage on the outside and dare BYU quarterback Taysom Hill, a much better runner than passer, to beat Texas through the air.

One of the biggest problems against BYU - in addition to Diaz basically playing a 4-2-5 scheme (this is what Texas opened in) and even a 5-1-5 scheme - was that the defensive line continued to shoot gaps instead of a base, read-and-react, gap control defense.

So when BYU would slant its offensive line to the left or right, and the Cougars did that a lot, it allowed BYU to get the Texas front flowing in one direction. Then, BYU would counter or cut back with Hill or a running back in the opposite direction with nothing but open field.

This is the same stuff that got Diaz's D in trouble last year. His D-linemen needed to hold their ground, engage blocks and shed blocks to protect their gaps. Instead, they were out of position all night.

Last night, Texas needed a solid base defense with an eight- or nine-man box and man coverage on the outside while daring Tysom Hill to throw. That didn't happen.

"Our performance on defense last night was unacceptable, and we need to change that," Mack Brown said in a statement. "Greg will be here tonight and get with the staff and players to start preparing for Ole Miss.

"He will be running our defense immediately. We're very fortunate that Greg has been around, watched all of our practice video and has a good scouting report moving forward.

"His familiarity with the staff and players should make for a smooth transition. He knows this place, did a terrific job in leading our defense before, and I'm excited to have him back on the field. We're back at it and working hard to beat Ole Miss this weekend."

Brown's choice of Robinson is being criticized in some circles because Robinson was fired from his last two jobs as head coach at Syracuse and as the defensive coordinator at Michigan under Rich Rodriguez.

But Robinson and Mack Brown worked well together at Texas in 2004, when the defense gave up a respectable 17.9 points per game and 107.4 yards per game on the ground (with an average of 3.2 ypc to opponents).

Some of Robinson's best work that season came in a 12-0 loss to Oklahoma, when Robinson led a fiery, impassioned effort against an explosive OU offense an held the Sooners to 12 points despite a couple short fields created by turnovers from the Texas offense.

Robinson also devised a game plan that befuddled a talented Texas Tech offense in a 51-21 victory that season.

"This is a tough deal for everyone involved, but I love The University of Texas, and Coach Brown has been wonderful to me," Robinson said. "I'll do anything I can to help him, so when he called, I told him I'd be there today. I know the staff, have a lot of respect for them and am excited for the challenge moving forward.

"I've been around the team a lot, watched all of their film and have been self-scouting and scouting opponents since I came on board in July. Because of Longhorn Network, I've also been able to see these guys for two to three years and am very familiar with the personnel."

This was a season Mack Brown vowed to fans would be one in which Texas could win all the games. Brown has been promising another run and that the Longhorns were about "to get good again." Being pulverized in Provo has now put the program in an incredibly negative light nationally with some pundits speculating it's only a matter of time before Brown steps down.

So Robinson enters a pressurized environment with a talented and undefeated Ole Miss (2-0) coming to town on Saturday.

"I need to get with the staff, work with them to get a plan in place and hit the ground running," Robinson said.

"I think very highly of the defensive coaches I'm working with - Duane (Akina), Oscar (Giles) and Bo (Davis) - so it's critical for me to get with them and figure out what specifically we need to do moving forward. Having coached here, I do know the ins and outs and having the stability with the rest of the staff will be very beneficial."

Robinson's personality during practices and games is almost polar opposite of Diaz. Robinson is fiery and in-your-face. He's not afraid to get after players and has had to game plan for quarterbacks like Peyton Manning in the NFL.

Mack Brown had a difficult decision after last season about whether to keep Diaz or seek a free agent like Robinson or Gene Chizik at that time. He opted to bring Diaz back, and after last night's wreckage, Brown had seen enough in a season in which Brown is trying to hold onto the fan base in a week-to-week relationship.

Time will tell if the move is enough to maximize the talent on the Texas roster and get things turned around - quickly.