It’s tough enough replacing a starting quarterback, let alone a legend like Peyton Manning.

Yet that’s the challenge facing another great QB, John Elway, who now runs the Denver Broncos team that Manning walked away from Monday.

“ Peyton Manning utilized every asset that God gave him to be the best football player that he could be,” Elway said. “And that, I think, to me is what sets him apart from anybody else because he got every ounce of ability that he has.”

Now, whether it’s Brock Osweiler, currently headed for free agency, or someone else who takes over behind center for the Broncos, Elway has an unenviable task.

History tells us that some Hall of Fame quarterbacks left their franchises in difficult circumstances, such as Johnny Unitas and Jim Kelly.

Others were barely missed: Joe Montana and Brett Favre.

And for a few teams, it was a mixed bag.

Successes

Three stand out in particular: Steve Young, Aaron Rodgers and Danny White.

Young was being groomed to take over for Montana even before Montana was ready to step (or be pushed) aside. And while Young didn’t lead the 49ers to four championships as Montana did, the team around him wasn’t as good as what Montana had. And the Cowboys built a powerhouse during the Young years.

Still, Young won two MVP awards and the 1995 Super Bowl.

Rodgers also has two MVP honors and a Super Bowl ring since replacing Favre in Green Bay. In fact, his seasonal numbers outdo Favre in many categories, and his three-year apprenticeship has turned out brilliantly for Cheeseheads everywhere.

White didn’t win any major individual awards after taking over for Roger Staubach in Dallas in 1980. He never got the Cowboys into a Super Bowl.

But he was a solid quarterback with a 62-30 record, a Pro Bowl appearance, and extra value in Big D as a punter.

Moderate Returns

When Elway set the bar for outstanding finales by winning the 1999 Super Bowl and then retiring, his replacement was Brian Griese. Not a bad choice, though comparing him with Elway was unfair from the outset.

A third-round draftee in ‘98, Griese had the misfortune of losing 2,000- yard rusher Terrell Davis to a massive knee injury during the QB’s first season in charge. Griese went 27-24 in Denver and was 45-38 overall in the NFL.

Elway’s contemporary, Dan Marino — perhaps the game’s greatest pure passer — left Miami in 1999. In stepped Jay Fiedler, who unlike Griese had three previous pro seasons as a backup.

Fiedler wasn’t outstanding, but he was no bust. In the next five seasons, the product of Dartmouth went 36-23 and twice led the Dolphins into the playoffs.

Flops

Jim Kelly guided the Bills to four straight Super Bowls; OK, Buffalo lost them all, but is anyone going to match that feat? Maybe not.

When the battered Kelly retired after the 1996 campaign, Todd Collins was his replacement. That lasted all of one year in which Buffalo was 5-8 and Collins had a QB rating of 69.5. He was gone the next year.

Manning’s idol, along with his dad, Archie, was Unitas, whose final season in Baltimore was an injury-ravaged 1972. Another Ivy League guy, Marty Domres of Columbia, fared so poorly (3-6, 9 TDs, 13 INTs) he was benched for rookie Bert Jones.