ATHENS, Ga. — Georgia star tailback Todd Gurley was suspended indefinitely Thursday while the school investigates an alleged violation of NCAA rules.

The stunning announcement came two days before the No. 13 Bulldogs (4-1, 2-1 Southeastern Conference) face No. 23 Missouri (4-1, 1-0) in a crucial Eastern Division game, a huge blow to both the team and Gurley’s Heisman Trophy hopes.

Georgia officials did not reveal the alleged violation. SI.com, citing an anonymous source, reported the school was investigating whether Gurley was paid $400 to sign 80 pieces of memorabilia this past spring.

The NCAA requires schools to declare players ineligible if they discover possible rules violations. They can apply for the player’s reinstatement after the investigation is completed, which happened in the past with stars such as Cam Newton and Johnny Manziel.

“I’m obviously very disappointed,” coach Mark Richt said in a statement. “The important thing for our team is to turn all our attention toward preparation for Missouri.”

Gurley leads the Bulldogs with 773 rushing yards and eight touchdowns, averaging 8.2 yards per carry. In addition, he has 11 receptions and a 100-yard kickoff return for a TD.

The junior established himself as a leading Heisman contender with several signature games and moments, including a school-record 293 all-purpose yards in a season-opening victory over Clemson. There was also a hurdler-like leap over a Tennessee defender to pick up an extra 10 yards, and last week the left-hander surprised Vanderbilt by completing a 50-yard pass — Georgia’s longest throw of the year.

The Bulldogs began the season with plenty of depth at running back, but that has taken a blow in recent weeks with injuries to top backup Keith Marshall and freshman sensation Sony Michel. Now, Georgia is dealing with the toughest blow of all.

Gurley is irreplaceable, a dynamic mix of speed and power that has helped the Bulldogs keep winning even with a mediocre passing game. He has accounted for more than half of the team’s rushing yards, and his nine touchdowns account for nearly a third of Georgia’s 30 trips to the end zone.

Freshman Nick Chubb, who ranks second on the team with 234 rushing yards, will likely step in to start against Missouri. Sophomore Brendan Douglas is the only other back with significant experience.

NCAA rules violations often involve the acceptance of impermissible benefits. Former Georgia star receiver A.J. Green was suspended for the first four games of the 2010 season after he acknowledged selling his Independence Bowl jersey to a former North Carolina player, who was regarded as an agent by the NCAA. Green had to repay the $1,000 he received, which went to charity, and the suspension dampened his final season with the Bulldogs. He went on to become a first-round pick of the Cincinnati Bengals.

Manziel, the Heisman Trophy winner from Texas A&M, was investigated before the 2013 season when reports surfaced that he had received money for signing autographs. The NCAA reached an agreement with the school for Manziel to sit out the first half of the opening game against Rice, with no finding that the player did anything wrong. He is now a rookie with the Cleveland Browns.

Gurley, who battled injuries as a sophomore, was on pace for huge numbers in what most figured would be his final season with the Bulldogs before entering the NFL draft.

He has rushed for more than 100 yards in four games, the only exception being a blowout win against Troy State in which he carried the ball only six times for 63 yards. Gurley had a career-best 208 yards against Tennessee and was coming off a 163-yard, two-touchdown performance in last week’s victory over Vanderbilt.

“He’s been my Heisman choice since the season started,” Georgia cornerback Damian Swann said after the Tennessee game. “I’m pretty sure he’s about to put one heck of a campaign together.”

That campaign has taken a big hit.