In two tries this season, they haven't had an answer for the Denver Broncos, either.
Manning threw for 403 yards and five touchdowns, four of them to Decker, and the Broncos held off a furious rally to beat the Chiefs 35-28 on Sunday and seize control of the AFC West.
Decker had eight catches for a career-high 174 yards for the Broncos (10-2), who moved a game clear of the Chiefs (9-3) in the division. With four games left, Denver also holds the tiebreaker by virtue two wins over Kansas City in the last three weeks. The Broncos have the AFC's best record.
Jamaal Charles had 93 yards rushing for the Chiefs, and capped a 17-play drive with a 1-yard touchdown run with 6:32 left in the game to close within a touchdown.
After forcing a punt with 3:32 remaining, Kansas City used three long pass plays to move deep into Broncos territory. But Alex Smith's pass to Dwayne Bowe in the end zone on fourth-and-4 with 1:45 left fell incomplete, allowing the Broncos to run out the clock.
Smith threw for 293 yards and two touchdowns for Kansas City, while Knile Davis returned a kickoff 108 yards for another score as the Chiefs jumped out to a 21-7 lead.
The Broncos promptly scored 28 straight points to take control.
Denver did it without of slew of injured players, including three starters on defense and tight end Julius Thomas, who has developed into one of Manning's favorite red-zone targets.
No need to worry. Decker more than shouldered the burden.
The fourth-year pro caught a 41-yard touchdown pass in the first quarter, two more TD passes in the third and a short fade for a score in the fourth quarter. Most of the time, Decker was abusing Chiefs defensive back Marcus Cooper, an undrafted free agent who'd become a starter.
Manning now has 41 TD passes, breaking his franchise record of 37 set last season.
The Broncos rebounded nicely from a disheartening overtime loss last week in New England, and are poised to get coach John Fox back on Monday. They went 3-1 under interim coach Jack Del Rio, who stepped in when Fox needed heart surgery during the team's bye week.
The Chiefs, who lost for the third straight week, squandered far too many chances on Sunday, including one right of the gate. They had marched deep into Broncos territory before Smith was picked off by Wesley Woodyard in the end zone.
Still, Kansas City seemed to have more energy in the first half.
The Chiefs took the first lead when Smith threw a touchdown pass to Junior Hemingway. And after Decker beat Brandon Flowers for a tying score, Davis took the ensuing kickoff deep in his own end zone and stepped out of two tackles before running untouched the rest of the way for a touchdown.
The 108-yard return was the longest in franchise history, and the first return touchdown by a Chiefs player since Jamaal Charles took a kickoff back against the Steelers during the 2009 season.
Smith's touchdown pass to Anthony Fasano, who made a nice one-handed grab, made it 21-7.
The Broncos closed within a touchdown later in the half when Manning hooked up with Decker for another long gain. This time, it was a short TD pass to Knowshon Moreno that capped the drive.
Manning began the second half off with a 37-yard touchdown pass to Decker, who made a nifty grab with his fingertips as he rolled into the end zone. Then after forcing a punt, the 12-time Pro Bowler hit Demaryius Thomas for 77 yards to set up Decker's 15-yard touchdown reception.
The Broncos' run of points ended when Decker caught a fade pass for a 35-21 lead. It turned out to be all the points they needed.