The Ole Miss defense is currently ninth in yards allowed and first in scoring defense nationally. Still, some people suggest that the defense will be better next season. I can't imagine they'll be statistically better, but they certainly shouldn't be much worse, if at all.
The Rebels lose safety Cody Prewitt, cornerback Senquez Golson (and his nine interceptions), cornerback Cliff Coleman, linebacker Deterrian Shackelford, linebacker Serderius Bryant, and linebacker Keith Lewis due to graduation (ETA: And defensive tackles Bryon Bennett and Lavon Hooks, as well as defensive end Carlos Thompson). While those are all contributors (with Golson and Prewitt being all-stars), none of them leave gaping holes the Rebels can't fill.
Sophomore Tee Sheppard was slated to start over Golson before suffering a season-ending injury in August. Assuming he returns at or near full-speed, there shouldn't be much drop-off from a coverage standpoint there He's not likely to equal Golson's interception totals, but he is rumored to be capable of locking down the best receivers opposing teams will throw at the Rebels. The other primary corner spots will feature Kendarius Webster and Mike Hilton, both of whom have proven to be very good so far. There are other corners like Kailo Moore waiting for their turn, and Ole Miss has a pair of cornerbacks in their current signing class who could vie for situational snaps immediately.
Freshman CJ Hampton was recruited by most of the SEC. In the end it came down to Alabama and Ole Miss, with both fighting or his signature. Hampton signed with Ole Miss and immediately began backing up Cody Prewitt. He was good enough that junior Chief Brown was able to move to Husky before suffering his own season-ending injury in August. Hampton hasn't played a huge role this season, but I'm sure some of that is based on backing up an All-American. He has tons of physical talent and could be a step up in terms of coverage. No one on the team will be able to match Prewitt's hitting ability, but Hampton isn't small and shouldn't struggle too much there either.
The key is the linebacker position. The Rebels lose a lot of contribution here. They'll still return Denzel Nkemdiche, but after that, there's a lot of unknown. Freshman Demarquis Gates has played well in limited snaps, and Christian Russell has the size needed at the position, but it's definitely a murky picture. The signing class currently features Leo Lewis, regarded as a top three player in state and a top five inside linebacker nationally. They're looking to add more quality players here, but I can't project who else they'll sign. It's something that needs to be shored up quickly, as it's the definite weak spot of the defense.
EDITED TO ADD SOME CONTRIBUTORS ON THE DEFENSIVE LINE:
Bryon Bennett is the best of the senior defensive linemen. He has really come into his own this year. Lavon Hooks plays some but isn't a major part of the rotation. While they both play a good bit, defensive tackle isn't an immediate position of concern. Robert Nkemdiche is obviously there, as well as Isaac Gross and Woodrow Hamilton. Freshman Herbert Moore is huge (though that's all we know so far), and Breeland Speaks, a freshman who is readshirting, appears to be the real deal.
As for Thompson, he has certainly gotten better since Freeze arrived on campus, but he's still not a disruptive player. I don't think he will be difficult to replace, at least somewhat.
Everyone else is back and a year older. I mean everyone. Other than the players I mentioned above the two-deep remains almost untouched. Assuming some players get better in the offseason, this side of the ball won't take a significant step back, and I think they could end up being a more complete defense, even if the statistics don't reflect that.