Of all the challenges that face teams in professional sports, the task of repeating as NFL champion can be stacked up with any of them.

That's because the deck is also stacked against Super Bowl winners, and has been ever since the 1993 collective bargaining agreement that put cement in the league's hard salary cap and made it very difficult to sustain great success from one season to the next.

Since Dallas repeated in 1993, the year the cap went into effect, there have been only two repeat champions. So, what happened six times in the first 26 years a back-to-back Super Bowl victory was possible — starting with Green Bay in 1967, the first opportunity — has happened just twice in the 25 years since (Denver 1997-98, New England 2003-04). It's not a coincidence.

When the Eagles open their attempt to beat the odds this season, beginning in earnest with the start of training-camp practices on Thursday, they will be some fighting some of the same forces that have derailed other attempts.

>> READ MORE: What you need to know about Eagles training camp

Just getting back to the Super Bowl is difficult enough. Only five teams have even done that since 1993, and in the last 20 years, more than 40 percent of all Super Bowl participants finished the following season with at least three fewer wins.

The league loves this, of course. Parity is what keeps hope alive in all 32 outposts of the empire. It is also what prevents anything resembling a dynasty from taking shape. New England is an outlier in recent NFL history, getting to the Super Bowl eight times in the Tom Brady era, but only two of those appearances came on the back end of consecutive seasons and only one resulted in a repeat championship. The Patriots are very nearly the exception that proves the rule.

The forces that create parity are obvious. Successful players get paid and, by definition, a championship team has a roomful of successful players. They can't all be paid by that team, however, and an upstairs-downstairs economy has developed on rosters. There are a handful of stalwarts, usually led by the quarterback, who eat up a majority of the cap and then a second-class of role players and reserves who aren't paid particularly well and, by and large, are poor replacements should the established, aging core suffer breakdowns of either ability or health.

>> READ MORE: What moves have the Eagles made in July and August in the last few years?

The Eagles are actually in the lucky position that their franchise quarterback, Carson Wentz, is still on his rookie contract and won't make serious money until the team exercises its fifth-year option for the 2020 season and his salary balloons to between $22 million and $30 million. That's when the real crunch will arrive, although the Eagles did have to shed contracts for this season and will have to do so again for 2019.

Check the recent history of the Baltimore Ravens for reference on this point. The Ravens won the Super Bowl following the 2012 season and had to give quarterback Joe Flacco his first big contract, a six-year, $120 million extension. As a result, the team was unable to keep its excellent defense together and struggled. In the five seasons since winning the championship, the Ravens are 40-40. They last made the postseason in 2014.

Even with the Wentz advantage, the Eagles needed to make significant adjustments coming off their Super Bowl win. In the offseason a year ago, according to overthecap.com, the Eagles only lost 12.2 percent of their snaps from the previous season, and had the third-most stable roster in the NFL from 2016 to 2017. This offseason has seen far more upheaval, with the departure of players who took 22.5 percent of the offensive and defensive snaps, almost exactly double the previous figure.

>> READ MORE: Press Taylor's 'exhaustive' approach to running the Eagles' quarterbacks room

It could be that the replacements will fit in just fine for those players who were integral parts of the championship. Cornerback Patrick Robinson played 866 regular-season snaps, including special teams, and led the team in interceptions, even taking one back for a touchdown in the NFC title game. If the Eagles are right, that void will be filled by redshirt Sidney Jones in conjunction with Jalen Mills and Ronald Darby.

In the same way, linebackers Corey Nelson or Nate Gerry can play the role of Mychal Kendricks (673 total snaps), receiver Mike Wallace can become Torrey Smith (737), Michael Bennett takes the defensive end rotation of Vinny Curry (642), tight ends Richard Rodgers and Dallas Goedert fill the places of Trey Burton (608) and Brent Celek (614), and on and on. Beau Allen (558), LeGarrette Blount (353), Najee Goode (493) and Corey Graham (551) served their purposes, but others can step in now. (Although no one would be surprised if Graham is brought back sometime during training camp.)

>> READ MORE: Nate Gerry didn't make an impact as a rookie, but they might need him to this year

What the front office has going for it as Howie Roseman's team works through this process is experience and the benefit of the doubt. It's a lot easier to know how to build a championship roster having just done it, and to sell both the locker room and the public on the alterations because of that resume.

Still, it's a lot of change, just as the NFL intends for it to be. It takes a great and a lucky team to overcome the roadblocks to repeating a Super Bowl victory. We know the Eagles were a great team last season and have the potential to be again. Now, we begin to find out if they will also be fortunate enough to select all of the right new faces to replace the old ones.

Repeat Super Bowl winners

  • 1967, Green Bay

  • 1973, Miami

  • 1975, Pittsburgh

  • 1979, Pittsburgh

  • 1989, San Francisco

  • 1993, Dallas

  • 1998, Denver

  • 2004, New England