As usual, the suspense has been taken out of declaration day. Monday marks the deadline for underclassmen to declare for special eligibility with the NFL ahead of the 2018 draft.

You can find the full list of underclassmen who have already done so right here. But who replaces the best of the best?

These are the 20 juniors you just don't replace. Sure, there are options at each of these schools. But there is also a reason these star players are coming out early.

They've done enough … without getting paid.

In alphabetical order, here's the top 20 underclassmen who have declared and who their respective teams hope will replace their production next season.

Josh Allen, QB, Wyoming: While Allen only went 16-11 as a starter, he led the Cowboys to the 2016 Mountain West title game. That means the edge of New Year's Six relevance. It looks like a battle will commence led by senior Nick Smith (3.9 GPA in finance), who filled in for a couple of games when Allen hurt his shoulder. Redshirt freshman Tyler Vander Waal will see the field for the first time, while early enrollee Sean Chambers, a three-star from Kerman, California, may even be in the mix. None of the three are getting comparisons, like Allen, to John Elway just yet.

Josh Adams, RB, Notre Dame: The best bets are senior Dexter Williams (9.2 yards per carry) or Tony Jones Jr. Featuring either one means replacing a powerful franchise back who briefly became a top Heisman challenger.

Mark Andrews, TE, Oklahoma: The loss of Baker Mayfield's favorite target is less about Andrews than replacing Mayfield himself. Kyler Murray will be throwing to a deep receiving corps. The next man up at tight end is sophomore Grant Calcaterra, who caught 10 passes as a promising freshman.

Saquon Barkley, RB, Penn State: There won't be another one like him for a long time. That doesn't mean Penn State isn't a top 10 program going into 2018. Rising junior Miles Sanders is more of a speed guy. James Franklin compared Sanders' arrival with Barkley in place to Aaron Rodgers coming in and learning under Brett Favre at Green Bay. High praise.

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Orlando Brown, OT, Oklahoma: Probably the key to Mayfield winning the Heisman when you consider his blindside obligations. Right tackle Bobby Evans will most likely move to left tackle to replace Brown. Evans was graded second in the Big 12 and top 10 nationally among tackles, ahead of even Brown, according to Pro Football Focus.

Sam Darnold, QB, USC: Reclassified freshman J.T. Daniels has a shot along with redshirt freshman Jack Sears. Daniels was already the No. 1 prospect in the 2019 class. Now Daniels -- the Gatorade national player of the year -- has a chance to be the guy this year if he follows through on his commitment. USC has had a bunch of success with Santa Ana (Calif.) Mater Dei products. Do Matt Barkley and Matt Leinart ring a bell?

Derrius Guice, RB, LSU: Leonard Fournette begat Guice who begat … who? The Next One at LSU has to be a difference-making power runner in Ed Orgeron's second full season as head coach. Three players -- none who ran for a total of 100 yards in 2017 -- have a shot: senior Nick Brossette, sophomore Clyde Edwards-Helaire and junior Lanard Fournette. Yes, he's the younger brother of Leonard. Four-star signee Chris Curry could also be in the mix. Clip and save for further use this fall. The Tigers lose 92 percent of their 2017 rushing total.

Sam Hubbard, DE, Ohio State: The Buckeyes' fearsome front takes a huge hit as Tyquan Lewis and Jalyn Holmes are also leaving. But only at Ohio State can that line be just as fearsome. Joey Bosa's brother Nick is the next great one after posting 8.5 sacks and 16 tackles for loss.

Josh Jackson, CB, Iowa: You don't just replace the nation's best defensive back. Iowa's All-American had freshman Matt Hankins starting opposite him. The other corner projects to be junior Manny Rugamba, who was suspended and lost his starting job last season. Iowa just seems to pump out difference-making defensive backs (see: Desmond King).

Lamar Jackson, QB, Louisville: How do you replace a Heisman winner? You don't. But Jawon "Puma" Pass -- that's really his name -- will try. As a freshman, the 6-foot-4, four-star from Georgia passed for 238 yards in mop-up duty. Reason to be optimistic: In that limited playing time, Pass actually had a higher passer rating than Jackson.

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Ronald Jones II, RB, USC: Only four players had more rushing touchdowns than Jones' 19 last season. Only four other Trojans have more career rushing yards. That's a big ask for the next man up at Troy. Freshman Stephen Carr averaged 5.5 yards per carry getting only 6.5 carries per game. Also watch redshirt junior Vaeve Malepeai and junior Aca'Cedric Ware. Together they combined for slightly more than half of Jones' 1,550 yards. 

Christian Kirk, WR, Texas A&M: One of the most productive college players in recent years, Kirk won't be replaced by one player. No other Aggie besides Kirk had numbers in all four all-purpose categories (rushing, receiving, punt return, kick return). The closest thing to it is tailback Trayveon Williams, the leading rusher and third-leading receiver.

Hercules Mata'afa, DT, Washington State: One of the best interior offensive line disturbers in the country, the All-American contributed to what was Mike Leach's best-ever defense. Redshirt junior Nnamdi Oguayo, another undersized lineman, started six games posting four sacks and seven TFLs.

Da'Ron Payne, DT, Alabama: Nick Saban covets that noseguard who can occupy two linemen in the A and B gap. Payne was that guy this season becoming both the Sugar Bowl and College Football Playoff National Championship defensive MVP.  Saban also loses Payne's backup in senior Joshua Frazier. Replacing Payne is a genuine concern with Saban looking at the 2018 recruiting class for help or rising senior Johnny Dwight.

Kamryn Pettway, RB, Auburn: The larger issue is how to replace one of the best 1-2 combos in the country? Gus Malzahn is also losing 2017 workhorse Kerryon Johnson. The Tigers won't be as deep at tailback Kam Martin was the next man up with 453 yards but only 16 of his 74 carries came when the score was within a touchdown either way.

Calvin Ridley, WR, Alabama: From Julio Jones to Amari Cooper to Ridley, Saban usually features an NFL-quality edge receiver who can stretch the defense. With Ridley gone to the NFL, look for Jerry Jeudy to fill it. Jeudy was a distant second this season in receiving with 14 catches averaging almost 19 yards.

Josh Rosen, QB, UCLA: Who knows who Chip Kelly wants with what is available on the roster? Redshirt sophomore Devon Modster is the quarterback with experience and had been backing up Rosen. Dorian Thompson-Robinson, an incoming freshman from Las Vegas Bishop Gorman, is a possibility. That's significant because Gorman runs an offense similar to Kelly's famous spread.

Bo Scarbrough, RB, Alabama: The biggest news may not be that that Scarbrough left but that rising senior Damien Harris stayed. Harris ran for exactly 1,000 yards and is within 1,300 yards of Derrick Henry's school record.

Roquan Smith, LB, Georgia: Whoever replaces Georgia's Will linebacker will be compared to one of the program's all-time best. Rising senior Juwan Taylor -- in Smith's recruiting class -- will likely get a shot but the competition will be incredible. Senior Natrez Patrick is expected back, but is he reliable after getting treatment for substance abuse? Sophomore Monty Rice has the most upside. Whoever it is, the next guy must have speed -- perhaps Smith's best trait. Any discussion of Georgia in 2018 starts with the linebackers with middle linebacker Reggie Carter also leaving.   

Vita Vea, DT, Washington: CBS Sports' Chris Trapasso believes Vea "might be the strongest prospect in the entire draft." Greg Gaines, a rising senior, is already a U-Dub defensive line starter but looks like the next great one. He graded out the fifth-best D-lineman in the Pac-12 and a top-20 nationally run stopper. Gaines missed only five tackles in 520 snaps.

Connor Williams, OT, Texas: The Longhorns got a good look at the future as Williams, a 2016 All-American, started only five games in 2017 due to injury (only 28 in his three-year career). Look for Elijah Rodriguez, an emerging talent, who missed the entire regular season with an ankle injury but then played in the bowl game. That may be enough. As Williams' replacement at left tackle, Rodriguez will be a central figure in Tom Herman developing a difference-making quarterback.