COLUMBIA — Jadeveon Clowney was not the only reason South Carolina had one of college football’s best defenses the past two seasons. The Gamecocks finished third nationally in yards allowed in 2011 and 11th last season. While the future No. 1 NFL draft pick played a major role on both teams, no one player can account for all the uncertainty the Gamecocks face in other areas of their defense entering this season.

Will Clowney’s pass-rushing speed from his defensive end position help new starters at linebacker and safety? Undoubtedly. It seems the only thing, other than injury, that can keep Clowney from having a special season would be a foolish decision to accept benefits from an agent. And he has not done that, according to the USC compliance department’s investigation into his reported contact with a sports agency run by the rapper Jay-Z.

With that issue seemingly resolved, of greater concern now to second-year defensive coordinator Lorenzo Ward is replacing all three of his linebackers from last season, as well as free safety D.J. Swearinger, the defense’s most vocal leader and hardest-hitting threat over the middle.

The Gamecocks begin preseason practices next Friday night at 7:15, with the first week or so of practices open to the public. As they try to build on back-to-back, program-best 11-2 seasons — success that owes largely to their defense — they must hope the young players now in prominent roles can produce like their predecessors.

At this point, the leaders at the mike, will and spur linebacker spots are sophomore Kaiwan Lewis, sophomore Cedrick Cooper and junior Sharrod Golightly, who is in a tight competition with redshirt freshman Jordan Diggs. Mike and will are USC’s two inside linebacker spots, while spur is a linebacker/safety hybrid position that requires more pass coverage.

Lewis is locked in at mike, the middle of the three spots, because he is physical enough to tackle ball carriers and able to perform the mike’s critical duty of aligning the defense before the snap, but “maybe not quite as athletic as some other guys” who can play the outside linebacker positions and roam the open field, said linebackers coach Kirk Botkin.

Ward and Botkin liked what they saw from sophomore Marcquis Roberts during spring practices, when he moved from spur to will. Ward said Roberts was probably his most impressive linebacker in the spring. Cooper would have been the starting will entering spring, but a knee injury sidelined him. Now that he is 100 percent, how he performs during the preseason could determine the positioning of USC’s wisest and most versatile linebacker.

Redshirt freshman T.J. Holloman has never played in a game, but is smart enough to play all three linebacker positions, and his coaches have raved about him. He is currently scheduled to rotate with Lewis at mike, but Botkin did not rule out a Lewis-Holloman starting combination at mike and will. He wants to work Holloman some at will during the preseason. But Ward said Holloman will enter practices exclusively as a mike.

“T.J. probably understands spur, mike and will as good as any young man I’ve ever been around,” Botkin said.

The reason Ward and Botkin have Lewis and Holloman at mike is because both are the sharpest at making pre-snap checks. And, as Botkin said, “There are some guys on our defense that rely totally on what the (mike) linebacker says to get lined up.” Botkin wants both of his best mike candidates prepared to play that role full-time if one of them gets injured.

In the secondary, the free safety has a similarly vocal presence, which is why Ward opted to move junior Brison Williams there, to replace Swearinger. Williams started at strong safety last season. Junior Kadetrix “J.J.” Marcus and sophomore T.J. Gurley, who have never started regularly, will move from free safety to strong safety, a position that better suits their inexperience and skills.

“The boundary (strong) safety has got to be a guy that loves to play in the box and be more physical,” Ward said. “That’s what T.J. Gurley is. That’s what J.J. Marcus is.”

Ward swapping the safeties means USC will have new starters at seven defensive positions when practices begin — six occupied by players who haven’t been regular starters. None must play like first-round draft picks for USC’s defense to shine again. But the sheer volume of new starters is a stark reminder that while Clowney can change many games, he alone can’t win them all.