You heard about Oklahoma's Landry Jones and his backup earlier this morning, but here are the most irreplaceable players for the rest of the conference.
Robert Griffin III, QB, Baylor: This one's pretty simple. Griffin means more to his team than any player in the conference. The 2008 Big 12 Newcomer of the Year helped the Bears win their 2009 season opener at Wake Forest before a season-ending knee injury in the third game. He'll come back for 2010 still a sophomore. But his injury last season now means his backup, Nick Florence, is surprisingly experienced.
Alexander Robinson, RB, Iowa State: Robinson rushed for 1,195 yards in 2009 and is by far the Cyclones' best player. His yards per carry average is almost 1.5 yards higher than his backup last season. This year, the battle for No. 2 is ongoing, with Beau Blankenship and freshmen James White and Jeff Woody trying to earn any spare carries not soaked up by Robinson.
Nate Solder, OT, Colorado: This season will be Solder's third as starting left tackle, the key position on the offensive line. He's proven to be one of college football's premier linemen, but his durability and experience have meant very little playing time for backup Ryan Dannewitz, a redshirt freshman.
Jake Laptad, DE, Kansas: Laptad is a force in the backfield and racked up 6.5 sacks and 12 tackles for loss last season. His backup is junior college transfer Quintin Woods, but with just four career tackles, there's a clear dropoff in both production and talent.
Blaine Gabbert, QB, Missouri: In just his second year as starter, Gabbert could be poised for a big jump like his predecessor, Chase Daniel. The Tigers earned a North title in Daniel's second season and No. 1 ranking after the regular season. But Gabbert needs to stay healthy. His backup is former walk-on Jimmy Costello, who's never played a meaningful snap, but behind him are a group of untested freshmen with potential in Ashton Glaser, James Franklin and Gabbert's younger brother, Tyler Gabbert.
Eric Hagg, S/LB, Nebraska: The central figure of the Huskers' Peso defensive scheme (Hey! Remember that?) gives the Husker defense the teeth that helped them nearly upset Texas and blow out Arizona. His backup is Austin Cassidy, who has plenty of on-field experience after appearing in all 13 games last season. Like Hagg, Cassidy has the ideal size for the position at 6-foot-1 and 210 pounds, and notched nine tackles as a sophomore in 2009.
Markelle Martin, S, Oklahoma State: Martin's sophomore season was overshadowed by big-hitting senior Lucien Antoine, but he'll be one of the team's leaders as a junior in 2010. He's the team's leading returning tackler, and should join Orie Lemon and Ugo Chinasa as the anchors of a defense replacing its four leading tacklers from 2009. His backup, Mathies Long, played in the last six games of 2009, but has just three career tackles.
Sam Acho DE, Texas: The better known of Texas' Acho brothers, he played in 24 games before taking over as starter last season. He notched 63 tackles and four fumble recoveries, tied for most in college football. He was also a semifinalist for the Lott Trophy. But at Texas, there's always a pretty narrow gap between starter and backup. Acho's backup should be either Russell Carter or Alex Okafor, who will also play behind opposite defensive end Eddie Jones. Carter played in nine games last season and notched five tackles. Okafor played in all 14 games last season and tallied 22 tackles.
Jerrod Johnson, QB, Texas A&M: No backup will be able to reproduce Johnson's impressive numbers from 2009 (38 touchdowns, 4,085 total yards). But Johnson's backup Ryan Tannehill does have plenty of experience--at a different position. He got plenty of reps this spring with Johnson sidelined from live action after minor shoulder surgery, but he's the team's active leader in receiving, with 1,418 career yards. He's thrown just nine passes in two seasons with the Aggies.
Colby Whitlock, DT, Texas: Should assume the role of nose tackle in Tech's new 3-4 scheme under coach Tommy Tuberville and defensive coordinator James Willis. Though it's a new position, Whitlock's experience will be tough to replace. Of his 46 tackles in 2009, 8.5 came behind the line of scrimmage. His backup is a mountain of a man, Myles Wade. The 6-foot-2, 340-pound junior college transfer made just two tackles in limited action last season, but he still has two years of eligibility left, and could plug plenty of holes in the middle of the defense after Whitlock graduates.