Oklahoma parted ways with long-time defensive coordinator Mike Stoops on Monday, announcing their dismissal of the assistant coach. News first of Stoops' departure first surfaced on Sunday night with multiple outlets reporting Stoops was either fired or resigning from his post. Oklahoma confirmed the former on Monday morning.

"I have great respect for Mike," said Sooners coach Lincoln Riley in a statement. "He's a quality football coach and a close friend. It became time for a change in our program and that happens sometimes in football, but we cannot lose sight of the accomplishments Mike had at Oklahoma. He was responsible for a lot of success and deserves the gratitude of everyone associated with Sooner football, not only for his role in the winning, but for coaching with integrity."

Stoops released a statement Monday afternoon thanking Oklahoma for his time as an assistant. 

Ruffin McNeill will take over defensive coordinator duties for the rest of the season, while Bob Diaco will move from being a defensive analyst to the outside linebackers coach.

The decision to part ways with Stoops comes on the heels of Oklahoma's 48-45 loss to Texas on Saturday in the Red River Showdown, the latest in a long line of poor performances from the Sooners defense. These performances were covered up by the explosive, high-scoring Oklahoma offense led by Kyler Murray this season and Baker Mayfield last year.

Following Oklahoma's loss to the Longhorns, its defense ranked 52nd nationally in yards allowed per play (5.29), but it's been the defense's inability to get off the field that has been the killer. The Sooners rank 98th in the country in third-down defense, allowing opponents to convert 42.4 percent of their third-down tries. In the red zone, things have been even worse as Oklahoma's opponents have scored points on 100 percent of their red zone possessions. That includes a touchdown-allowed rate of 85.7 percent, which ranks 124th in the country, tied with the likes of Rice and UAB.

But it's not just the stats, it's also the overall performance against good teams. Last season, the Sooners defense allowed 27.1 points per game, which wasn't good but it wasn't awful, either. However, against winning teams, that number spiked to 31.9. The story hasn't been any different in 2018 as the Sooners have allowed 27.3 points per game overall and 34 against winning teams.

Stoops had been Oklahoma's defensive coordinator since the 2012 season. He's the brother of former Oklahoma coach Bob Stoops, who retired before the 2017 campaign. Riley retained Mike Stoops and led the Sooners to a College Football Playoff berth last season. McNeill, Riley's long-time mentor, was his former boss at East Carolina.

Oklahoma is off this week but plays at TCU on Oct. 20.