The Patriots dynasty had just started to bud, and Patrick Mahomes had yet to escape elementary school. But the paradoxical advice Manning delivered Delhomme would ring true for the next 15 years, and it still holds up now, as Mahomes and Kansas City Chiefs Coach Andy Reid become the latest quarterback-coach combo to face Belichick in the playoffs, in Sunday’s AFC championship game.

Manning told Delhomme, in so many words: Be prepared for the unexpected, but don’t overthink. Playoff opponents have been trying — and mostly failing — to straddle that line ever since.

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“That’s because of the mystique the Patriots have created, and what they create in other peoples’ heads,” Delhomme said this week. “The question Mahomes is going to be facing is, ‘Are you going to be prepared for the unknown?’ That’s the mystique the Patriots bring, and it’s been earned.”

Coaches and quarterbacks who prepared for Belichick in the playoffs have mostly met the same dismal fate. Under Belichick, the Patriots are 28-10 in the playoffs. No other franchise in that span has played more than 25 playoff games or won more than 15. “This isn’t their first rodeo,” Reid said this week.

Belichick and the Patriots present myriad challenges, but the central one is that opposing coaches know Belichick will devise a game plan specifically for them. They just don’t know what it will be, or even at what point in the game a new wrinkle will be deployed.

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“Here’s what I would say: Expect the unexpected, if that’s possible,” said Brad Childress, who coached under Reid in a Super Bowl and a playoff game against the Patriots. “You always say, ‘Don’t block ghosts,’ but you better be ready for all eventualities.”

Against the Patriots, diligence can be a trap. Some players like to study film on their own, to pick up on opponent tendencies. Against most teams, an inspection of their previous four or five games will reveal pre-snap alignments and shifts that indicate what will happen after the snap. The Patriots force opponents to disregard, or at least be wary of, recent history. The Patriots will do something before the snap that looks identical to what happened on film, but the play itself will be entirely different.

“You do have to educate your football team,” said former New York Giants defensive coordinator Steve Spagnuolo, whose Super Bowl XLII game plan helped derail New England’s undefeated season. “Do your job, rely on your fundamentals, because they could do anything. They’re going to do things you haven’t seen yet.”

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Former Indianapolis Colts coach Tony Dungy, now an NBC Sports analyst, faced Belichick three times in the playoffs and beat him once, in the 2006 AFC championship. He realized that no matter how much recent film he watched of the Patriots, it would not reveal Belichick’s plan. “Most people in the playoffs do what they always do,” Dungy said. “They do what’s gotten them there. He’s not tied to that. If they’ve been riding Tom Brady and they’ve been hot for three or four weeks but your run defense is suspect, then running the ball 40 times is okay for him.”

The possibilities could cause coaches to overthink their preparation, and Dungy developed an approach for the Patriots. The Colts would make their best game plan, but then enter a game willing to change on the fly, with three or four specific contingencies in mind.

“We had to tell our team, you don’t know what attack you’re going to get,” Dungy said. “You don’t know what defense you’re going to get. You need to hang in there and weather the storm and then you say, ‘This is what they’re going with.’ It becomes a chess match, and you’ve got to be flexible.”

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In a 2008 divisional game in New England, the Jacksonville Jaguars advanced into Patriots territory with less than five minutes left, trailing 31-20, in position to make it a one-score game. In studying the Patriots’ safeties all week, quarterback David Garrard had seen only two pass coverages from them in their previous games: Cover 3 or Cover 1, meaning the safeties defended a specific portion of the field. All game, the Patriots had given Garrard exactly what he expected. On fourth down, Garrard dropped back and saw wide receiver Matt Jones sprinting into a vacant spot in the defense. Or so he thought.

“Rodney Harrison drops out of the sky,” Garrard said. “I’m thinking he’s playing Cover 3 or normal Cover 1. He comes down, reads my eyes, intercepts the pass. I got to the sideline and said, ‘Where did he come from?’ ”

The Jaguars only solved the mystery the next day, reviewing the game on tape. Harrison had played a “lurk position,” Garrard said, drifting in the secondary and reading Garrard rather than defending a zone. It had worked because the Patriots, Garrard said, had not shown that defensive strategy on film all season.

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“They’re going to add some wrinkles to their defensive schemes that you’re just not going to be prepared for,” Garrard said. “Once you see it, you’re probably behind the 8-ball. . . . And then they have another wrinkle to go behind that in-game. Their in-game game plan changes.”

In the two weeks before Super Bowl XXXIX, Childress, the Eagles’ offensive coordinator under Reid, developed a game plan for a 3-4 defense, which Belichick has deployed his entire coaching career. When the game started, the Patriots came out with a 4-3 front, and the surprise defense wreaked havoc with Philadelphia’s protections. The Eagles went scoreless in the first quarter.

Just knowing Belichick’s tendency to surprise can have adverse effects. Coaches and quarterbacks can get lost in their own heads trying to discern what Belichick will do, and it takes them out of simply playing to their strength.

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“It’s called chasing ghosts,” Garrard said. “You do that sometimes.”

John Fox, who lost to the Patriots in the Super Bowl with Carolina and beat them in an AFC title game with Denver, would devote extra study to situational football. He knew Belichick would know every detail about his team, so Fox would instruct his staff to research the Patriots’ fourth-down plays, two-point plays and other specifics against opponents similar to his team. The trick was to distill the extra information to players without overloading them.

“You only give players so much,” said Fox, now an ESPN analyst. “You want to make sure you’re practicing the right things — but not too many things, because that’s screwing them up.”

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The Patriots offer a mental gauntlet beyond strategy. Leading up to a 2016 divisional-round game in New England, Chiefs wide receiver Jeremy Maclin had battled a bum ankle. As Maclin ran through a pregame workout to test the injury, Childress, then an offensive assistant under Reid, noticed something unusual. Steve Belichick, Bill’s son and defensive assistant, and Ernie Adams, Belichick’s mysterious right-hand man, had crept all the way down to the Chiefs’ end of the field to inspect Maclin’s movement.

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“They’re all looking at something,” Childress said. “You don’t know what they’re looking at.”

In a 27-20 Patriots victory, Maclin caught two passes for 23 yards.

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The Patriots’ reputation for information-gathering, of course, sometimes crossed over into alleged malfeasance — most notably through the Spygate scandal of a decade ago. That fosters paranoia — and extra work — for some teams. Mahomes and the Chiefs will be playing at home, which means they don’t have to worry about one preparation factor visitors to Gillette Stadium sometimes have.

Garrard said the Jaguars would prepare for coaches’ headsets suddenly malfunctioning. They practiced using hand signals or running substitutes in from the sideline to call plays, and Garrard learned 10 plays he could call at the line of scrimmage if he lost contact with coaches.

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“Every time we played in New England, whether it was the playoffs or regular season or preseason, the headphones always went off — always,” Garrard said. “It’s automatic.”

The best way to combat a coach who offers surprise is to try to surprise him. In Super Bowl XLII, one of the greatest upsets in playoff history, Spagnuolo made several small but important tweaks to his scheme. On one blitz, he showed an alignment that all season had meant one defender blitzed and the other dropped into coverage, but reversed their roles. On a crucial third-and-one, the Giants gave a look that he believed would cause quarterback Tom Brady to call an off-tackle run, and when he did, they stopped it. They rolled coverage to wide receiver Randy Moss, but not too much, because doing one thing repeatedly would allow Brady to shred them.

“More than anything, our guys willed themselves to win that day,” Spagnuolo said. “Bill Belichick is still Bill Belichick. We just happened to win that particular day.”

In Delhomme’s Super Bowl, the Panthers nearly toppled the Patriots, scoring 19 points in the third quarter and losing on a last-second field goal. During the week, offensive coordinator Dan Henning told the Panthers, “They’re going to try to give us something we haven’t seen. We’re going to be the aggressor.” Carolina ran the ball in the first and third quarters to wear down the Patriots’ loaded defense, then spread out and ran a two-minute style passing offense in the second and fourth.

Henning also tried to chip away at Belichick’s aura. The Patriots had not yet become a colossus, but Belichick’s status as a football genius had crystallized. Henning had known Belichick for years, having worked with him on staffs under Bill Parcells. In Panthers team meetings all week, Henning called Belichick “Little Billy” to his players.

“Honestly, we had a comfort in our minds,” Delhomme said. “He would tell us, ‘Just keep sticking with what we’re doing.’ ”

The Chiefs may be perfectly suited to counter Belichick. Their multitude of offensive weapons gives them opportunities to break tendencies without sacrificing getting the ball to a talented skill player. Mahomes has faced Belichick once, and his ability to make plays outside the pocket will limit how creative Belichick can be.

Mahomes has proven unshakable all season. “You don’t throw 50 touchdown passes getting wide-eyed,” Childress said. That will be tested again Sunday, not only by Belichick, but by the opposing quarterback.

"They’re going to throw some new wrinkles at you, so be alert. If throwing the ball away is your best play, be willing to do it,” Garrard said. “Playing Brady makes you feel like every pass must be completed, and must gain yards. It doesn’t have to. You have to be willing to let the game come down to the last drive.”

Reid may be even less likely to cower than Mahomes. His teams are 2-6 all-time against Belichick, but since he came to Kansas City, the Chiefs’ offense has mostly dominated New England, including in a 43-40 Patriots victory earlier this season. Reid’s creativity will challenge Belichick as much as Belichick challenges him.

“There’s a million things that you could find in any of those years, and who knows if that’s the one he’s going to pick out against you, so I think you have to put your chips on something,” Belichick said this week. “You have to try to do something that you feel like is important in the game, try to do that well, have a plan for how you’re going to deal with other things and let the game declare a little bit, which it will. … I don’t think trying to read his mind is really that beneficial.”

Over two decades, opponents have realized the same thing about Belichick. The Chiefs will be the latest team to try to prepare for the unexpected without overthinking. It sounds like a nearly impossible task, and that is just how Belichick wants it.