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Reaching the College Football Playoff takes a tremendous amount of skill and a healthy bit of luck, but injuries are affecting a few of the top contenders for the Football Bowl Subdivision’s championship.

Although coaches around the country preach “next man up,” some players lost to injury simply aren’t easily replaced — especially if a national title is a realistic goal. The respective programs aren’t eliminated from the discussion, but early-season injuries have made the road to Glendale considerably more difficult.

Clemson

Shortly before the 2015 campaign started, Clemson lost a starter in defensive back Korrin Wiggins. Then, during the season opener, star receiver Mike Williams — who tallied 1,030 yards and six touchdowns last year — exited the game after sustaining a head injury when he slammed against a protective pad.

Clemson is a slightly under-the-radar contender, but the offense needs every weapon available to navigate a tough upcoming schedule that includes No. 6 Notre Dame and No. 20 Georgia Tech.

UCLA

Josh Rosen’s emergence seemed to solidify UCLA’s presence among college football’s potential elite, but the injury bug had a different idea. In a matter of just three weeks, the Bruins have lost a standout performer at every level of the defense. Eddie Vanderdoes (knee), Fabian Moreau (foot) and Myles Jack (knee) are each gone for the year.

UCLA’s championship aspirations now ride on the throwing arm of a true freshman quarterback and the legs of junior running back Paul Perkins. The Bruins will test the “next man up” theory Saturday when they challenge Arizona and the nation’s No. 5 offense on Saturday night.

TCU

Between the departures following the 2014 season and injuries sustained this year, TCU is teetering on the edge of a defensive catastrophe.

James McFarland (foot) led the Horned Frogs with seven sacks in 2014 but hasn’t played. Starting linebacker Sammy Douglas (undisclosed), safety Kenny Iloka (knee) and cornerback Ranthony Texada (knee) are all on the shelf for the season. Combine those with Mike Freeze’s leave of absence and Mike Tuaua’s suspension, and TCU is relying on its explosive offense.

At least Heisman Trophy favorite quarterback Trevone Boykin is still healthy — Horned Frogs fans, go ahead and furiously knock on wood. However, because nothing can be easy for the Big 12 program, five receivers are battling various nicks and dings. TCU’s final two games of the season are against early-season offensive juggernauts Oklahoma and Baylor. Good luck with that.

Notre Dame

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Perhaps no other college football team has suffered more losses to key players than Notre Dame. The list simply goes on and on — and on and on. Avery Sebastian (foot) has been sidelined since the first game, but at least he’s set to return in a few weeks. Other Irish weren’t so lucky.

Starting quarterback Malik Zaire (ankle), running back Tarean Folston (knee) and tight end Durham Smythe (shoulder) each won’t return in 2015. Starting defensive tackle Jarron Jones (knee), nickelback Shaun Crawford (knee), safety Drue Tranquill (knee) share the same fate.

Notre Dame has done an admirable job replacing those six key starters, and the remaining schedule is certainly favorable. But if the Irish ultimately fall short, it’ll be easy to wonder what could’ve been if injuries didn’t plague the College Football Playoff-worthy roster.