NFL fans love a good rivalry, and for much of the 21st century, they were treated to a great one between two Hall of Fame quarterbacks. From the 2001 to 2015 seasons, Tom Brady and Peyton Manning faced each other a whopping 17 times, including four times with a trip to the Super Bowl on the line.

This week's AFC Championship Game in Kansas City features arguably the league's two best young quarterbacks in Patrick Mahomes of the Chiefs and Josh Allen of the Buffalo Bills. Neither signal-caller is older than 25. Both are leading MVP candidates this season. The two combined to throw for 9,284 yards and a staggering 75 touchdown passes.

And while it will be a while before we can compare Allen vs. Mahomes to Brady vs. Manning, it's not hyperbole to say we could be in the nascent stages of a rivalry under center that will dominate the AFC—and the NFL—for the next decade.

The two have met once—a Monday night affair in Week 6 of this season that Mahomes and the Chiefs won 26-17. In that inaugural duel, neither quarterback lit up the scoreboard. But Mahomes had the better game, completing 21 of 26 passes for 225 yards and two scores. That game was scheduled for Thursday, October 15, but was moved to the following Monday night because of the COVID-19 pandemic.

There probably won't be any moving of this second meeting. But there's a massive cloud hanging over the game nonetheless: Mahomes' status after he landed in the concussion protocol when he took a shot in last week's win over the Cleveland Browns.

There was potentially some good news in that regard Monday, courtesy of Carrington Harrison of CBS Sports Radio in Kansas City:

ESPN's Adam Schefter noted that Mahomes is expected to practice Wednesday, but Schefter also tweeted a reminder that Mahomes suffered a foot injury in the first half of the divisional-round win that many forgot about once he left the game.

Mahomes will still need to pass a series of neurological tests ahead of Sunday's game. But for the sake of argument (and football fans everywhere), we'll assume that Mahomes' injuries aren't serious enough to keep him off the field Sunday at Arrowhead, and that he'll be (reasonably) close to 100 percent.

It's a relative number by the second half of January.

If that's the case, we could be in for an all-time classic of a game.

It took Mahomes all of three seasons to go from surprise pick out of Texas Tech to the gold standard at the NFL's most important position. In his second year in the league, Mahomes became just the second player (after Manning in 2013) to throw for 50 touchdown passes and 5,000 yards in the same season. That year, Mahomes was named the league's Most Valuable Player and led the Chiefs to the AFC title game before they fell in overtime to the New England Patriots.

The next season, Mahomes would avenge that setback, leading three straight double-digit comebacks in the playoffs on the way to winning the Super Bowl LIV MVP award.

This season, his fourth, Mahomes threw for 4,740 yards—second only to Deshaun Watson of the Houston Texans. He led the league in passing yards per game (316.0), finished fourth in the NFL with 38 touchdown passes and led the Chiefs to a 14-2 record and a third consecutive conference title game at home—a feat that had never been accomplished in the AFC.

As Rany Jazayerli wrote for The Ringer in November, Mahomes has already accomplished as much (if not more) in January or later than some quarterbacks who are considered mortal locks to be enshrined in Canton when their careers are over:

"While Mahomes has a short postseason resume so far, he has more heroics in five playoff games than most quarterbacks do in their entire careers. He already has more playoff wins after trailing by double digits (three) than Rodgers does (two). Mahomes has averaged 295 yards per game in the playoffs, a hair off his regular-season average of 304, and his touchdown-to-interception ratio of 6.5 is better than his regular-season mark of 5.2—even though the Chiefs have played from behind in four of the five games and the defenses generally knew what to expect."

Remember, he's also just 25 years old.

Josh Allen, 24, hasn't had the consistent success that Mahomes has. But if his 2020 breakout campaign was any indication, the third-year pro out of Wyoming is just getting started.

After a pair of up-and-down seasons to begin his career, Allen got a shiny new No. 1 receiver in 2020 in the form of Stefon Diggs—and the volume was cranked to 11. After he completed less than 60 percent of his pass attempts each of his first two years, Allen's completion percentage spiked to 69.2. He threw more touchdown passes (37) in 2020 than in his first two seasons combined. Allen's passer rating jumped by over 20 points to 107.2. And he led the Bills to a 13-3 record, the AFC East title and the team's first berth in the conference title game since Buffalo's four straight Super Bowl trips in the early '90s.

While speaking with Vic Carucci of the Buffalo News, Allen credited an offseason of working on his mechanics with his improvement this season:

"Being able to add my hips and make that as consistent as possible and try to slow everything else down up top and use my hand as the leverage for the speed and the accuracy has changed a lot of things. The accuracy has gone up, but it's actually added some mph to my throwing power, too. It's been a pretty cool process. … It was like a wake-up call."

Whether it was Diggs' arrival, improved mechanics, Allen's natural maturation or a combination of those factors, he went from a talented but erratic quarterback over his first two seasons to a legitimate MVP candidate in year three. His improvement has been remarkable, and there's nothing to indicate it was a one-off.

Now the two quarterbacks with arguably the strongest arms in the game will battle for a trip to Tampa, Florida, in a contest that has shootout written all over it. Last summer, Mahomes made it clear to ESPN's SportsCenter (via Adam Teicher and Marcel Louis-Jacques) that he's not about to cede anything to Allen—including the title of biggest gun in the NFL:

"Obviously, Josh has an extremely strong arm. But I've yet to see someone have a stronger arm than me, so maybe we can line up—I know we talked about having a throw-off—and then we can prove who really has the strongest arm. I have ultimate belief in myself. He does have a strong arm, but I've put it out there 80, 85 yards. If he beats that, he beats it."

The stakes are slightly higher this week.

There aren't a ton of parallels that we can draw between Brady vs. Manning and Allen vs. Mahomes—for starters, the youngsters are exponentially more athletic than Brady and Manning ever thought about being. It's also premature to compare Mahomes and Allen to two quarterbacks who have combined for 13 Super Bowl appearances and eight championships.

But the Brady vs. Manning rivalry defined AFC football for 15 years. Seventeen meetings total (of which Brady won 11), four AFC title games (of which Manning won three).

There's a long way to go before Allen vs. Mahomes can be mentioned in the same breath. Never mind the possibility that 2019 MVP Lamar Jackson could insert himself into the battle to be the battle at quarterback. Jackson is the youngest of the lot and immensely talented.

But Jackson also hasn't gotten past the divisional round (yet) and just got bounced from the playoffs by Allen and the Bills. Regular-season matchups are well and good, but it's the contests with the biggest stakes where rivalries go from good to great.

Where we are right now, it's Allen and Mahomes who most have the combo of talent inside them and weapons around them to dominate the conversation under center in the AFC for a similarly extended period as Brady and Manning—much to the despair of the rest of the conference.

Sunday we'll find out which one gets bragging rights in the first go-round between the two with a trip to the Super Bowl on the line.

And we could all be bearing witness to the beginning of an epic struggle for dominance of the AFC between two outstanding players whose best days (we hope) are still to come.

As Terrell Owens would say, "Get your popcorn ready."