Mike Leach's Air Raid offense didn't disappoint in its SEC debut on Saturday as visiting Mississippi State upset defending champion LSU 44-34 in Baton Rouge, and quarterback K.J. Costello set a conference record for passing yards in a single game with 623.

The previous record was held by Eric Zeier of Georgia, who threw for 544 yards against Southern Miss in 1993.

"I'm speechless about everything that happened today," Costello said. "If you were to tell me a year ago today that I'd be sitting here, it's crazy."

Costello came to Mississippi State this offseason as a graduate transfer after starting 25 games in three seasons at Stanford. His previous high for passing yards in a game was 381.

Costello's 623 passing yards against the No. 6 Tigers was the 11th most in FBS history, and the most in a player's debut. His 60 pass attempts is the most by a Bulldogs quarterback.

"I thought it was a really good debut," Leach said.

Asked to rank Costello's performance against past quarterbacks with whom he has worked, Leach said, "It's really high when you consider playing at LSU against the defending national champions."

Leach lauded Costello's composure. He said Costello's ability to bounce back from multiple turnovers was as good as any player he has seen.

LSU, which lost 17 starters from the team that went undefeated last season, became the first defending national champion to lose its season opener since Michigan in 1998.

"We have no excuses," Tigers coach Ed Orgeron said.

Orgeron said that losing All-American cornerback Derek Stingley Jr. to an illness "really hurt us" and was compounded by the fact that his replacement, Jay Ward, missed two weeks himself while recovering from an injury.

Another defensive back, Darren Evans, transferred from FCS Nicholls State less than three weeks ago.

Orgeron's message to the young defensive backs who played: "Welcome to the SEC. You've got to get better."

Orgeron said he was surprised how out of sync the offense was. Myles Brennan, who took over for Heisman Trophy winner Joe Burrow at quarterback, threw for 345 yards, three touchdowns and two interceptions.

No Tigers running back ran for more than 50 yards.

"I told the team we're going to find out what we're made of," Orgeron said. "Let's go back to work."

LSU is scheduled to go on the road to play Vanderbilt next weekend, and Mississippi State is slated to play its home opener against Arkansas.