AUSTIN, Texas — Texas fired defensive coordinator Manny Diaz on Sunday, less than 24 hours after one of the worst defensive performances in the history of the program. Longhorns head coach Mack Brown announced that former Longhorns defensive coordinator and Syracuse head coach Greg Robinson will take over the job. Texas’ defense under Manny Diaz has picked up where it left off last year. Cooper Neill / Getty Images ”Our performance on defense [Saturday] night was unacceptable, and we need to change that,” Brown said. BYU ran for 550 yards, the most ever allowed by Texas, in a 40-21 victory against the Longhorns, putting a potential breakthrough season on the brink of another big disappointment if the Longhorns can’t fix their defensive problems. Texas (1-1) plays No. 25 Mississippi (2-0) at home on Saturday. Brown hired Diaz after Texas finished 5-7 in 2010 and defensive coordinator Will Muschamp left to be the head coach at Florida. The hire was a surprise considering Diaz had been defensive coordinator just one season at Mississippi State, but Brown called him a rising star in coaching who brought energy and new ideas to his struggling program. Aside from a blowout loss to Oklahoma, Diaz’s first season was a success as Texas matured into the best defense in the Big 12. But the unit regressed into the one of the worst in program history in 2012. As the season wore on, players complained that teammates weren’t lining up correctly or playing hard every game. Texas hid some of those problems with a 9-4 finish and a bowl victory and Brown shrugged off questions if Diaz would be replaced. But after the loss to BYU, Diaz’s tenure was being measured in minutes. Television cameras during the game showed Brown huddled with Diaz and defensive backs coach Duane Akina seemingly pleading with them to find a way to stop BYU’s marauding rushing attack. Brown refused to say in his postgame news conference if he planned to keep Diaz on the job, and by Sunday afternoon announced that he will be reassigned within the athletic department. Diaz makes more than $600,000 a year. Brown knew he was taking a chance on Diaz when he hired him. Diaz, who once worked at ESPN and thought he wanted a career in sports journalism, was inspired to pursue coaching after helping cover a Bill Parcells news conference.