KANSAS CITY, Mo. -- The NFL placed the game between the Kansas City Chiefs and Indianapolis Colts in this week's premium TV window, Sunday night, to showcase what could have been the top quarterback matchup of an era, Patrick Mahomes against Andrew Luck.

But Luck abruptly retired from the Colts this summer, saying constant injuries had taken away his love for the game, leaving Jacoby Brissett as the Colts' starter. With apologies to Brissett, who is playing well and has a passer rating of more than 100, it's difficult not to feel a sense of loss, not just for Sunday night but for the foreseeable future.

The careers of AFC QB heavyweights Philip Rivers, Ben Roethlisberger and Tom Brady are winding down, which would have opened the door for a Mahomes-Luck rivalry in the 2020s that had the potential to equal the Brady-Peyton Manning rivalry of the early 2000s. Not anymore.

"As a fan, any time one of the best players walks away from the game, it always leaves a bad taste in your mouth," Chiefs safety Tyrann Mathieu said. "You want to watch them further and see what they could become. Obviously, there were other dynamics that went into Luck's decision.

"As a player, any time you get to compete against guys like Andrew Luck, Tom Brady, Drew Brees, you always remember those games. You like to play against those guys. From playing against Luck, I know what kind of competitor he is and I know how great he could have been."

Mahomes, the NFL's reigning MVP, is off to another spectacular season. He's on pace to throw for more than 6,000 yards, which would break the NFL record.

Mahomes beat Luck in last year's playoffs in what will be their only meeting unless Luck eventually returns.

"That would have been a sight to see, two top-caliber quarterbacks in this league," Chiefs cornerback Bashaud Breeland said. "From a defensive-back standpoint, I'd like to see Andrew Luck as well. With the things he did in this league, that would be a great challenge. But he's not here so I'm just focused on Jacoby Brissett."

Brissett, with 10 touchdown passes and two interceptions, is 11th in the NFL in passer rating.

"From a perspective of competitiveness and poise, I just think he's passing his first test here with flying colors," Colts coach Frank Reich said. "Nothing's too big for him. He seems most poised in the biggest moments. Physically he's showing his accuracy is very good and his ability to extend plays is very strong. He's mobile in the pocket. And then mentally he's handling our offense really, really well and doing great in the run game with what we ask him to do. As far as pass progression stuff, [he's] really sharp."

Mathieu faced Brissett in 2017, when the quarterback replaced the injured Luck and Mathieu played for the Arizona Cardinals.

"He's going to take care of the football," Mathieu said. "You won't see him trying to force throws. He's tough to tackle. I'd probably compare him to Roethlisberger in the pocket. He knows how to make guys miss, especially bigger guys, defensive linemen.

"What's different between Luck and Brissett is that Luck is going to take his chances. He's going to take his shots down the field. Brissett is definitely more careful in that sense."

With Luck out, the most logical AFC quarterback for Mahomes to build a rivalry with is Houston's Deshaun Watson. The Texans play the Chiefs next Sunday, also at Arrowhead Stadium.

That game will be played in the afternoon and not at night, a decision the league perhaps regrets.

"I truly believe Deshaun Watson is going to give you the same thing over the years that Luck would have," former Tampa Bay Buccaneers general manager Mark Dominik said. "He's the quarterback people are still sleeping on a little bit. He has elite ability from an athletic standpoint, arm talent, intelligence, leadership."

Dominik mentioned Derek Carr of the Oakland Raiders and Lamar Jackson of the Baltimore Ravens as other top candidates capable of challenging Mahomes for AFC quarterback supremacy over the next few years. Dominik's list also includes Baker Mayfield of the Cleveland Browns and Sam Darnold of the New York Jets.

But none of those quarterbacks is the sure thing that Luck seemed to be.

"It's too bad Andrew isn't playing but everybody respects the reason why," Chiefs backup quarterback Matt Moore said. "A rival for [Mahomes] will pop up somewhere where you least expect it. That's the way it is. You lose one potential rivalry but another one will come along and everyone will be fired up about that matchup."