Back in January, on the couch at home, Ron Rivera was watching the Kansas City Chiefs cling to 22-17 lead over the Cleveland Browns with 1:17 left in the fourth quarter. It was fourth and inches near midfield. Kansas City Coach Andy Reid, one of Rivera’s most valued mentors, kept his offense on the field. Reid looked down at his play sheet; a trip to the AFC championship game was on the line.

“Knowing Andy, I wouldn’t put it past him to the throw the ball,” Rivera remembered saying to Stephanie, his wife.

“Really?” she replied. “Would you [scheme against] him for that?”

Passing was even riskier than usual. Kansas City’s star quarterback, Patrick Mahomes, was out because of a concussion. Reid could have given the ball to running back Darrel Williams, who was averaging six yards per carry, or backup Chad Henne, who had thrown 30 passes in the past six seasons — as well as an interception on the previous drive.

“Probably not,” Rivera said. But then his mind drifted to Veterans Stadium, Sept. 12, 1999.

Rivera was a first-year linebackers coach under Reid, the Philadelphia Eagles’ rookie head coach. In their first game, the Eagles led the Arizona Cardinals 24-22 and faced third and four near midfield with two minutes to go. Everyone seemed to expect Reid to run, so he passed — and it almost worked. The receiver caught the ball but fumbled, and the Cardinals kicked a game-winning field goal as time expired.

Twenty-two years later, as Henne rolled right — a pass! — Rivera shook his head. The call crystallized to him Reid’s daring and unpredictability, why it is so hard for even his former assistants to game-plan against him, which Rivera has done this week with the Washington Football Team hosting Kansas City on Sunday.

Rivera and others from the 1999 Eagles’ staff, which produced eight head coaches and helped establish Reid as the NFL’s most influential modern coach, say decades of insight can mean little during games because Reid’s only true play-calling tendency is a lack of one.

“That’s him,” Rivera said of Reid’s pass against Cleveland, which worked. Rivera imagined himself on the opposite sideline and wondered whether, in the never-ending game of “I know that you know that I know,” Reid would suspect he remembered Arizona in 1999. “If he sits there and says, ‘I think he knows I’m going to throw it, so I’ll run it.’ You just don’t know with him. That’s what keeps you guessing.”

On Sunday, Rivera will be the third former assistant Reid has faced in this season’s first six weeks. Reid has fared well against them overall, with a 11-5 record, according to TruMedia, but he has lost both times this season, first to John Harbaugh’s Baltimore Ravens and last week to Sean McDermott’s Buffalo Bills. Rivera is 1-1 against Reid, and mutual friends joked Rivera should call Bills defensive coordinator Leslie Frazier, with whom Rivera played on the Chicago Bears, to ask the finer points of how he stifled Mahomes with two-deep zone coverages.

“I’m proud of them,” Reid said of his coaching tree, even as it might be conspiring against him. “I don’t like getting beat by them, but I’m proud of them. It bothers me, period, losing games. I’m not big on it. But listen, they’re good coaches.”

In this way, Reid echoes Mike Holmgren, his mentor, doppelganger and link to pro football’s history as part of the Bill Walsh coaching tree. Holmgren coached Green Bay and Seattle and was the Reid of his generation. He thought of his former assistants as sons, particularly Reid, because, as he joked, “He copied my mustache, but mine’s a little neater.”

Holmgren said players came to understand the tension he felt facing his guys, and some, such as Reggie White and Brett Favre, teased him for getting so uptight.

“They said, ‘You’re acting a little weird,’ and I’d get mad. ‘No, I’m not!’ ” Holmgren remembered. “ ‘[They said]: ‘Yes, you are! Just admit it!’ ”

If there’s one unifying theory among Reid’s assistants, it’s that his success has come from hard work and adaptability. He maintains the same routines year after year and blends innovative play-calling with superior talent. When facing him, Rivera paid close attention to personnel groupings — Reid believes in “players, not plays” as much as anyone — but knew his mentor self-scouted to avoid predictability.

Brad Childress, who coached with Reid in Philadelphia and Kansas City, noted Reid has boxes of notes on his game card called “Get-you-going” plays. They’re specific calls to help Mahomes, if he falls out of rhythm, or to feed elite talents such as tight end Travis Kelce or wide receiver Tyreek Hill.

“That’s the great thing about Andy,” Childress said. “He’s not a dinosaur. He may have come up in the dinosaur age, but he has changed every year, and he’s amenable to suggestions.”

Sometimes, though, nothing works like an old favorite. In January 2008, Childress led the Minnesota Vikings against the Eagles in a wild-card game and knew his mentor would, at some point, use the slow screen. They had “overemphasized” it against early-down blitzes in Philadelphia because their quarterbacks, running backs and linemen excelled at the timing and feints necessary to sell it.

Midway through the fourth quarter, holding a 16-14 lead, the Eagles had first and 10. Reid called the slow screen, and even without pressure, running back Brian Westbrook took it the distance.

“In my wildest dreams, I didn’t think it was going to go for a [71]-yard touchdown,” Childress said, adding, with a laugh, “I kind of wanted to stick my foot out and trip Westbrook as he ran by me on the sideline.”

Childress added what distinguishes Reid in the insular world of NFL coaching, what makes it hard to coach against him, is his innovation.

In 2016, he had joined Reid in Kansas City, and they were scouting quarterbacks even though they didn’t plan to draft one. Reid was watching tape on North Dakota State quarterback Carson Wentz when he saw a play he liked, a creative one with three verticals to the short side and the running back going up the middle seam. He scribbled it on one of the 3-by-5-inch note cards he keeps on his desk.

In the spring, Kansas City tweaked the play’s elements to fit its personnel. Reid polished it in the preseason, and the week before the season opener against the defending champion New England Patriots, it made the game plan. Early in the fourth quarter, the Chiefs were deep in their own territory, trailing 27-21. Reid called “All Go HB Seam.”

Running back Kareem Hunt beat the linebacker, and quarterback Alex Smith hit him in stride for a 78-yard touchdown. Kansas City took the lead to stay. Patriots Coach Bill Belichick could have watched every play Reid had ever run, been an assistant for years, and never seen it coming.

“It was an unscouted look, a look they hadn’t seen,” Childress said.

The challenge of beating a mentor, especially one as successful as Reid, is important to Rivera. He cherishes football legacy and history, and when he thinks about how he spends most of his time, he often sees Reid’s influence. The game will matter a little more to him Sunday.

“It means something to me personally that I came up with Andy Reid, that he gave me my first chance,” Rivera said. “He basically put me on the road to where I am today.”