Shackelford was injured during a routine drill late Monday afternoon in the eleventh of Ole Miss' 15 allotted spring practices, when he suddenly fell to the turf of the team's indoor practice facility.
Shackelford attempted to get up but couldn't, and was seen repeatedly slamming his helmet against the turf. Trainers helped Shackelford off the field. Shackelford, who totaled 48 tackles last season, had been often praised by the Ole Miss coaching staff for his leadership this spring.
"Everything starts with him due to his leadership," defensive coordinator Tyrone Nix said following Saturday's full-contact scrimmage. "He's challenging himself and his teammates to play at a higher level and to be more consistent."
"I feel like I am (a leader)," Shackelford said in an earlier interview with the Ole Miss Spirit. "Not in a cocky way, still being humble. At the same time, I recognize I do have a big role on this team, and I know that players will follow me. If you don't have good leaders, you're really not going to have a good team."
A starter at weak-side linebacker, Shackelford's absence leaves a sizeable void alongside middle linebacker Mike Marry and strong-side linebacker Joel Kight. Kight had minor surgery on his knee today.
Sophomore Clarence Jackson, junior Rudy Wilson and redshirt freshman Ralph Williams are the notable players remaining who could be called upon to take Shackelford's place in this final week of spring drills.
Ole Miss signed Philadelphia, Miss., linebacker C.J. Johnson -- rated a four-star prospect by Scout.com -- in February.
Johnson, a U.S. Army and Parade All-American, totaled 149 tackles and two interceptions as a senior for Philadelphia High School. The 6-foot-2, 235-pound Johnson was also named first team All-State by the (Jackson) Clarion-Ledger. He's of great importance now, if he wasn't already.
"They're going to line me up everywhere when I get there," Johnson said, also in an earlier interview with the Ole Miss Spirit. "I could be playing some inside linebacker, outside linebacker or even defensive end. They want me to roam free and make plays. We're going to turn it around in a hurry. We're going to change the defense completely around next year."
Shackelford led the team in sacks with five last season. He totaled 20 tackles, two for loss, in his true freshman season in 2009. Shackelford will presumably redshirt in 2011-12, with two years to play two following his return to the field.
"D.T.'s high school coach told me he was the best player he'd ever had -- on the practice field, in the community, in the classroom. A lot of coaches say that, but it's true with him," Ole Miss head coach Houston Nutt said Friday.
"He proves it every day. I'm really proud of him, especially with several seniors out with injury. He's taken over the leadership role."
Nutt was somber in his meeting with local media Monday. Asked if Shackelford's injury was serious, Nutt said it "could be," but wanted to wait until after the MRI to comment further. Ole Miss is scheduled to practice at 4 p.m. this afternoon.
"D.T. is second to none when it comes to leadership," Nix said. "The first thing you (look for) when you're defining a leader is a guy who leads by example, and he's a guy who does it on and off the field. He's an excellent example of a student-athlete. He's never satisfied with his performance because he understands there's always room for improvement.
"I think his leadership is going to be very critical on this team and on this defense, and he's headed in the right direction."
Now Shackelford will have try and lead from the sidelines, Tuesday a defining moment in a season not yet off the ground.