In this week’s Thursday 10 we take look at the most irreplaceable players in college football.

It should come as no surprise that this list is quarterback-heavy. Quarterback is the most important position in football, and this group reflects that. But don’t be surprised when you don’t see some of the sport’s best players on this list. Many of them play for annual contenders, and talent is much easier to replace on rosters like those. That’s why you won’t find players like Saquon Barkley (Miles Sanders) or J.T. Barrett (Joe Burrow/Dwayne Haskins) or Minkah Fitzpatrick (pick an Alabama DB) with highly-touted backups.

The players below are a mix of first-round-level talent and the ability to potentially wreak their team’s season if something happens to them.

If these players go down, a team’s chance to contend goes with them.

Lamar Jackson, QB, Louisville: Of course the 2016 Heisman Trophy winner is on this list. Jackson played on a different level from every other player in the country for most of last season. He accounted for 51 total touchdowns, which outpaced 78 FBS teams. Jackson threw for 3,543 yards, ran for 1,571 and confounded defenses at every step. The scary part is he could get even better. Jackson took a major step forward as a passer in 2016 but still completed just 56.2 percent of his passes. If Jackson becomes more efficient with his arm, he’ll be practically unstoppable. It’s also worth noting Jackson’s backup is untested redshirt freshman Jawon Pass after Kyle Bolin elected to transfer. If Jackson gets hurt, Louisville’s season is sunk.

Sam Darnold, QB, USC: Want to know how important Darnold is to the Trojans? Once he took over as the full-time in Week 4 last season, the Trojans went 9-1 and averaged 35.9 points per game. In the three games Darnold didn’t start, USC went 1-2 and averaged 20.3 points per contest. So yeah, Darnold makes a difference. Darnold finished with 3,086 yards, 31 touchdowns and a 67.2 percent completion percentage as a redshirt freshman, and he should only improve in 2017 as a second-year starter. If USC hopes to finally return to college football’s upper echelon, it’ll be Darnold who leads it there. He’s a likely Top 10 pick in next year’s NFL Draft, and he’s inarguably one of college football’s most irreplaceable pieces.

Derrius Guice, RB, LSU: When Leonard Fournette went down with injury last year, Guice stepped in and the Tigers run game did not miss a beat. But if Guice, a junior, gets injured this season, LSU won’t be nearly as fortunate. Guice, at 5-foot-11, 212 pounds, is a potent blend of natural ability and meanness. He runs with an angry purpose, which resulted in 1,387 yards and 7.6 yards per carry a year ago. That’s after he averaged 8.5 yards per tote as a true freshman in 2015. There are plenty of highly-touted former recruits behind Guice, there's just nobody left on LSU’s depth chart that can make a Guice-level impact. Guice is a likely Heisman contender for an offense that features a “game manager” at quarterback in Danny Etling. New offensive coordinator Matt Canada should put Guice in the best position for success. If healthy, the results should be spectacular.

Quinton Flowers, QB, South Florida: The most important piece for the AAC favorite, Flowers threw for 2,812 yards and rushed for 1,530 yards a year ago. He proved equally as efficient as both a runner and a passer completing 62.5 percent of his passes (24 TDs vs. 7 INTs) and averaging 7.7 yards per carry, which ranked seventh nationally. A third-year starter, Flowers has improved significantly every season he’s been in Tampa Bay. If that growth continues, the Bulls’ first ever conference championship will be in reach. Oh yeah, Flowers’ backup, sophomore Brett Kean, has thrown just 26 career passes. South Florida needs Flowers to play well and stay healthy in a conference that's as deep as they come from year to year.

Mason Rudolph, QB, Oklahoma State: A senior and three-year starter, Rudolph is the caretaker of an offense that could put Oklahoma State in College Football Playoff contention this year. A 4,000-yard passer a year ago, Rudolph threw 28 touchdowns against just four interceptions while averaging 9.13 yards per attempt. He’s one of college football’s best deep ball throwers and the point man for arguably the nation’s most balanced offensive attack. The Cowboys are looking for their first Big 12 title since 2011. If Oklahoma State manages to topple rival Oklahoma, it will likely be because of Rudolph. By the way, Rudolph’s current backup, Taylor Cornelius, is a walk-on. That’s how important Rudolph is for the Cowboys.

Ed Oliver, DT, Houston: The Cougars lost three defenders to the NFL Draft last season, but their defense should remain fearsome in 2017 thanks to Oliver’s presence. 247Sports’ True Freshman of the Year in 2016, Oliver finished with 19.5 tackles for loss, 61 total tackles, five sacks and nine pass breakups as a defensive tackle. At 6-foot-2, 290 pounds Oliver is among the nation’s best pass rushers from the interior. His real strength, though, is anchoring a Houston run defense that finished fourth nationally a year ago. Oliver isn’t a 350-pound 0-technique, but he's so strong in his lower body that he can easily occupy two blockers. Pro Football Focus ranked Oliver fifth nationally against the run a year ago among DTs.

Harold Landry, DE, Boston College: The nation’s sack leader (16.5) returns for his senior season, which is critical for a Eagles defense that has to go up against offenses like Clemson, Louisville, Florida State and Notre Dame. Landry is arguably the top edge rusher in the nation, but he’s also one of the best run stuffers in the country – Pro Football Focus ranked him fifth in the country among DEs a year ago. Landry is Boston College’s best player without question, and he’ll have to remain at an All-American level in order for the Eagles to compete in one of college football’s toughest divisions.

Derwin James, DB, Florida State: The Seminoles already know what it’s like to be without James, and it ain’t easy. A true freshman star in 2015, James played nearly every defensive position for Florida State in a year where it finished ninth nationally in points allowed. Last year, James suffered a season-ending injury in Week 2. From that point on the Seminoles allowed 25.7 points per game, which would’ve ranked the team 48th nationally for the entire season. There are other factors that impacted the defensive drop off from 2015 to 2016 (like a lack of quality linebacker depth), but there’s no question Florida State’s defense is significantly less flexible without James. The junior is the most versatile player in college football, and his skill set cannot be replaced.

Baker Mayfield, QB, Oklahoma: We’re breaking our rule putting Mayfield on this list – former five-star Kyler Murray is his backup – but that’s how good the Heisman finalist was a year ago. Despite the Sooners running the ball on 60 percent of their possessions, Mayfield threw 3,965 yards, 40 touchdowns and just eight interceptions. Most impressively, Mayfield averaged 11.1 yards per attempt (the only quarterback to be above 10.5 for the year) and completed 68.2 percent of his passes. The senior does nearly everything well, and he’s the key to Oklahoma’s national title chances after the offense lost a Heisman finalist (Dede Westbrook), the team’s all-time rushing leader (Samje Perine) and an all-purpose monster (Joe Mixon).

Shane Buechele, QB, Texas: Buechele isn’t as naturally talented (or proven) as the rest of the players on this list, but that doesn’t make him any less important to Texas’ chances in 2017. The Longhorns have searched for a quarterback answer since Colt McCoy walked off the field against Alabama in the 2009 national title game. The result? A 46-42 record and three straight losing seasons. New head coach Tom Herman has a workable point man with Buechele for his power-spread attack. But if he goes down, the result would be disastrous for Texas. The Longhorns have only two scholarship quarterbacks on their roster, and Buechele’s backup is a true freshman (Sam Ehlinger) coming off an injury-plagued senior season. Texas, with plenty of talent, is primed for a potential breakout this year with Buechele on the field. If he isn’t, Herman’s first campaign could collapse quickly.