The Aggies make their SEC debut boasting a defense that led the nation in sacks in 2011. But they’re breaking in a new quarterback, hardly a recipe for success against SEC defenses.

Here are three position battles to monitor in fall camp:

1. Quarterback

Top contenders: Jameill Showers, Johnny Manziel, Matt Joeckel

The battle to replace Ryan Tannehill is an open competition but Showers, a redshirt sophomore, has emerged as the favorite. He has the strongest arm, and as the backup last season he completed 4 of 5 throws for 40 yards. At 6-2, 219 pounds, Showers is the strongest QB in Aggies history: he squatted 575 pounds as a freshman. Manziel, a redshirt freshman, is a Parade All-American who threw for 45 TDs against five INTs as a high school senior. His status is unclear due to a June arrest, though he’s practicing. Joeckel, a sophomore, has good pocket presence and ideal size at 6-4, 234 pounds.

2. Wide receiver

Top contenders: Kenric McNeal, Mike Evans, LeKendrick Williams, Nate Askew

Seniors Ryan Swope and Uzoma Nwachukwu return to lead the WR corps, but there’s still room for one or two more starting wideouts in the spread attack run by Kevin Sumlin and coordinator Kliff Kingsbury. At Houston last season, Sumlin’s offense led the nation with 599.1 yards per game. McNeal missed the final eight games last season due to injury but is ready for a big senior year. Evans (6-5, 218) is a redshirt freshman who flashed big potential in spring practice and continues to shine in fall camp. Williams is a 5-foot-8 speedster who who returned 11 kicks and punts for TDs in a single high-school season.

3. Strong safety

Top contenders: Howard Matthews, Steven Campbell, Clay Honeycutt

Matthews played in every game as a true freshman last season, starting twice, and had 28 tackles and two sacks. But he’s suspended for the first game due to a violation of team rules. That could open the door for Campbell or Honeycutt. Campbell, a 6-foot, 201-pound senior, is a difference-maker when healthy. He returned an interception 51 yards against SMU last year and made four tackles against Oklahoma State.

For more up-to-the-minute news and analysis from SEC bloggers Daniel Lewis and Larry Hartstein, follow @CBSSportsSEC.