By Jason Keidel

It takes years for most quarterbacks to prove their salt as NFL starters. And once the get there they tend to evolve, with two glaring exceptions.

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Robert Griffin III and Colin Kaepernick have plunged from the orbit of instant stardom to benchwarmers who aren’t starters, or even coveted by the clubs that drafted them.

Griffin was way more celebrated coming out of college. After winning the Heisman at Baylor, the athletic QB landed in Washington DC after owner Dan Snyder famously (or infamously) traded the team’s entire draft board to get him. And Griffin lived up to the billing, winning Offensive Rookie of the Year and the NFC East, and leading the Redskins to a rare playoff position. He assumed a handle commensurate to his status — RG3.

But in his first playoff game, a loss to the Seattle Seahawks, Griffin shredded his knee and hasn’t been the same on the field, or off. The Seahawks game was as renowned for the wretched field conditions of FedEx Field, which left RG3 rendered in a fetal position, clutching his knee in agony.

Then whispers floated about his being difficult to deal with. More rumors were murmured about Snyder regarding Griffin like a movie star, lavishing him with planes, trains, automobiles and bodyguards.

There’s a long history of rookies beaming from the marquee before they earn their stripes. Like Joe Namath, who signed a deal upward of a half-million dollars in 1965. But it took one challenge of fisticuffs in the huddle and an equally blue-collar toughness on the turf to prove his worth.

Fair or not, Griffin has an inverted reputation. He’s become the emblem of the coddled star who slithers to the bench at the first hint of trouble. Add to that the change of coaching guard. Gone was Mike Shanahan, who drafted and developed RG3. Enter Jay Gruden, who made it clear that while Griffin may not be discarded, he wasn’t regarded as royalty. Gruden often publicly lamented the quarterback’s inability to read defenses or even take the proper, 3 or 5-step drop in the pocket.

After going 9-6 in his maiden season, Griffin went 3-10 as a sophomore. His total QBR plunged from 75.63 to 42.16 between 2012 and 2013. He threw 20 touchdowns in 2012. He threw four last year. He tossed a paltry four interceptions in ’12. He threw five last year, in eight fewer starts. After going 9-6 his rookie year, he’s since gone 5-15. There isn’t one element of his professional life that isn’t swirling down the NFL drain.

Since then, the Redskins have gone with everyone but their prized QB, and have now settled on Kirk Cousins, who had a fabulous 2015, throwing for over 4,000 yards and leading the Skins to the playoffs. Washington is so confident in Cousins, they’re trying to carve out a long-term contract, which has left RG3 professionally lonely, sans a starting job, if not a home franchise. There’s no sense that the Redskins want Griffin anymore, in any capacity.

But since half the league is forever starved for a starting quarterback, Griffin will at least get the chance to make a roster, if not start. A yearly roll call of forlorn franchises needs help at QB, with Cleveland often leading the way. But no matter where RG3 lands in 2016, it was impossible to imagine all of this three years ago, at the scalding start of his career. For someone who has no rap sheet, Griffin’s rep could not be worse.

Then we have Colin Kaepernick, who entered the league in 2011 with much less fanfare. Picked by the San Francisco 49ers in the second round out of Nevada — hardly a hotbed of NFL talent — Kaepernick was a safety blanket for longtime starter Alex Smith. Until he wasn’t.

After Jim Harbaugh made the controversial decision to bench Smith — who took the Niners to overtime of the NFC title game — Kaepernick flowered. He led the 49ers to the playoffs in his first year as a starter, then took San Francisco to the Super Bowl. The 49ers lost to the Baltimore Ravens.

Like Griffin, Kaepernick was seen as the tip of the new QB spear, heading the wave of nouveaux, hybrid quarterbacks who could conquer you through the air or on the ground. At 6-foot-5, 230 lbs, armed with a howitzer for a right arm and absurdly long legs that could cover five yards in one step, Kaepernick played like a halfback who could throw the ball 70 yards without breaking stride, or a sweat.

Kaepernick ran the read-option with epic fury and epic results, gashing NFL defenses every Sunday, leaving gassed defenders pawing for his jersey while he galloped into the end zone again and again. Perhaps his magnum opus was the playoff game against the Packers. Kaepernick stampeded through Green Bay for an NFL-record 181 yards. It was the greatest rushing game for a QB in league history, in the playoffs or regular season. He averaged a surreal 11.3 yards per carry, including 20-yard and 56-yard jaunts that left all-world linebacker Clay Matthews gazing at the ground in defeat.

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Ironic that just two years later, Matthews was seen sacking Kaepernick in San Francisco. In the new stadium, a victorious Matthews stood over the fallen Kaepernick, and sarcastically barked, “You’re no Russell Wilson.”

Kaepernick had a historic season in 2013, throwing for nearly 3,200 yards, 21 touchdowns and just 8 interceptions, with a passer rating of 65.93. He also rushed for four more scores, averaged 5.7 yards per rush and led all NFL QBs with 524 yards on the ground. Yet his fall from grace and face of the franchise was as steep as Griffin’s.

There are myriad reasons Griffin crumbled. His mangled legs took forever to heal. Changes in coach, personnel and injuries all helped to derail the bench-dwelling QB.

But Kaepernick hasn’t been profoundly injured. He’s never been arrested, hasn’t tested positive for PEDs or narcotics. He merely forgot how to play football. No doubt a decaying roster didn’t help, but his spiral down the rungs of relevance were about more than retired teammates and free-agency defections.

Was he a Harbaugh contrivance, a football float that burst once his coach bolted for Michigan? While we all have a relative grasp on the physical bona fides of an NFL quarterback, the metaphysical bond between quarterback and head coach is more opaque. How much of a high-end quarterback is nature versus nurture?

If the last 15 months are any indication, Kaepernick was a Harbaugh baby, hatched by the madman and mad scientist who’s succeeded Bill Parcells as the coaching revenant. No one in college or pro football is better at pumping the proper gridiron joules into a foundering franchise.

He leaves San Francisco and they instantly falter. He arrives at Michigan and they instantly improve. Harbaugh will overstay his welcome — even at his alma mater — because that’s what he does. For all his coaching splendor, Harbaugh is, by all accounts, an unbearable control freak who has no filters or sense of diplomacy. It’s all part of the package.

Enter Chip Kelly, perhaps Kaepernick’s final football lifeline. Kelly is renowned for a binary approach to quarterbacking, equal parts surf and turf. Kelly is so enamored with the Oregonian approach to quarterbacking he was rumored to have lusted for former Heisman QB Marcus Mariota, who wound up in Tennessee and seems to have the talent and temerity to prosper in the NFL, despite his laconic approach and comically sedate personality.

So perhaps Kelly sees Colin as Mariota-Lite, if not Mariota-like. Now that he’s floundered over the last year or so, the media and the masses have taken a jeweler’s eye to Kaepernick’s personality.

Once lauded as an enthusiastic kid who plays the game with appealing joy, Kaepernick has gone from fun to foul, a kid with way too much hubris to be a leader. The celebrated TD ritual — kissing his robust biceps — is now disparaged and discouraged, as the emblem of immaturity.

The two former star quarterbacks have so many surreal similarities. Both were part of the young QB invasion of all-world athletes who don’t live in the bubble of the pocket. Both lost the coaches who drafted and believed in them. Both were replaced by coaches who didn’t. Both QBs have cultivated a reputation as ornery employees who now do more harm than good. Funny how those traits always coincide with a dip in on-field production.

Because of the dearth of dominant quarterbacks this (and every) year, Colin Kaepernick and Robert Griffin III will get more chances, even if not with their first NFL clubs. They were too talented at one time to be tossed onto the recycle bin of former stars.

No matter how bad things look today, they will still spin through the revolving door of at least one more franchise, even if they’re no longer franchise players.

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Jason writes a weekly column for CBS Local Sports. He is a native New Yorker, sans the elitist sensibilities, and believes there’s a world west of the Hudson River. A Yankees devotee and Steelers groupie, he has been scouring the forest of fertile NYC sports sections since the 1970s. He has written over 500 columns for WFAN/CBS NY, and also worked as a freelance writer for Sports Illustrated and Newsday subsidiary amNew York. He made his bones as a boxing writer, occasionally covering fights in Las Vegas, Atlantic City, but mostly inside Madison Square Garden. Follow him on Twitter @JasonKeidel.