Grant, who was expected to be Choice's replacement Thursday night against Virginia Tech, was knocked out of the fourth quarter of last Saturday's win over Army after taking a hit above his right ankle.
He had rushed for 119 yards and a touchdown before leaving with the injury. The team said Friday that tests showed it was more serious than first believed.
The injury denies Grant the opportunity to make his first start since his 2004 freshman season.
Grant has 39 carries for 231 yards and one touchdown this season.
"I feel bad for Rashaun," coach Chan Gailey said. "It's tough to lose guys, especially now, when he was going to get an opportunity to start next week."
Georgia Tech is off this weekend.
Choice had arthroscopic surgery on Tuesday for a right knee injury and is expected to miss at least a week. Freshman Jonathan Dwyer and sophomore Jamaal Evans will likely get the bulk of the snaps against the Hokies.
AUBURN: The Tigers are holding out hope that this wacky, upset-filled season will somehow end with them getting a shot at a Southeastern Conference championship.
Mississippi, on the other hand, just wants to finally win a league game.
The two teams meet today, trying to rebound from tough blows to their seasons. The Tigers lost in the last seconds at No. 3 LSU, and the Rebels remained winless in the SEC, losing 44-8 to Arkansas.
It holds a different kind of significance for the Tigers (5-3, 3-2), who will need to win their final three SEC games and get plenty of help from other teams to win the Western Division.
TENNESSEE: The Colquitt family will donate $100,000 to their alma mater to endow a football scholarship, athletic director Mike Hamilton said.
"The Colquitts have made a mark on Tennessee football for more than 20 years," Hamilton said. "This gift ensures they continue to make an impact for years to come."
Tennessee's "first family of fourth down" has produced four top punters for the Vols - Craig Colquitt (1975-77), his brother Jimmy (1981-84) and Craig's sons Dustin (2001-04) and current punter Britton.
Jimmy and Dustin were all-Americans, while Craig and Britton made all-Southeastern Conference. Jimmy remains Tennessee's career punting average leader at 43.9 yards, followed by Britton at 42.9, Dustin at 42.6 and Craig at 42.5.
VIRGINIA: Week after week, the Cavaliers have appeared to find themselves in to-the-final-minute fights. And Virginia has kept winning, whether at Scott Stadium or on the road.
"That's a skill that certain teams have," coach Al Groh said. "It's like anything that happens repetitively after a while; it's not just an accident."
That formula has worked well enough for No. 21 Virginia so far, taking a team that looked awful in its season opener to seven owins.
Now the Cavaliers - after No. 8 Virginia Tech's 14-10 loss to No. 2 Boston College on Thursday night - find themselves atop their division in the ACC.