Andy Reid's coaching tree is vast and includes five current NFL head coaches: the Chicago Bears' Matt Nagy, Baltimore Ravens' John Harbaugh, Buffalo Bills' Sean McDermott, Philadelphia Eagles' Doug Pederson and New York Giants' Pat Shurmur.

Now Reid, coach of Kansas City Chiefs, matches up against Nagy, who was around Reid for a decade with the Chiefs (2013-17) and Philadelphia Eagles (2008-12).

The two head coaches squared off during the 2018 preseason, but this game counts. And familiarity will be a theme Sunday, as the Bears like to do a lot of the same things the Chiefs do.

"They'll know a lot about us and what we do, particularly offensively," Reid said. "Likewise, in return, we'll know them."

Chiefs defensive coordinator Steve Spagnuolo said his defense must be prepared for anything and everything.

"The great thing about those systems that you're talking about is sometimes when you prepare for them, you don't get what you prepare for or what you're trying to copy," Spagnuolo said. "They're really good at attacking what you do or don't do.

"So, for us, it's going to be as much as finding out in that first quarter early on what Matt's deciding to do, how he's going to attack. That will be the game within the game."

Nagy's team faces a tall order, considering Reid's regular-season success against former assistants. In these master-vs.-pupil scenarios, Reid is 11-6 against former staffers, including a 5-4 mark against ex-assistants the first time he's faced them.

"I know one of those five wins was against me," Spagnuolo said with a chuckle.

In 2011, Spagnuolo — then head coach of the St. Louis Rams — played Reid's Eagles in the season opener and lost 31-13.

Reid's other wins in first-time meetings against former staffers came against Pederson in 2017, then-Jets head coach Todd Bowles in 2016, then-Browns head coach Shurmur in 2012 and then-Vikings head coach Brad Childress in 2007.

Reid's four losses? They came against Bills head coach Sean McDermott in 2017, then-Panthers head coach Ron Rivera in 2012, then-Vikings interim head coach Leslie Frazier in 2010 and Ravens head coach John Harbaugh in 2008. (Notably, Harbaugh, has lost four straight games to Reid since then.)

Spagnuolo admits to feeling some anxiety before squaring off against Reid the first time, but attributed some of that to the game being a season opener and dealing with the high expectations for that year's Rams.

But Spagnuolo said each situation — and each former Reid assistant — is unique.

"Everybody treats it a little bit differently," Spagnuolo said of playing your former boss.

A high level of respect remains between Reid and Nagy. Ask one about the other, and you better get out of the way or risk getting run over by flood of high praise.

"He's smart," Reid said of Nagy. "He's passionate about the offensive side of the ball. He's especially passionate about football, but he's involved with the offensive side. He's very creative.

"You have to expect anything. He'll do it. He's not afraid to do things, so you have to make sure that you study and that you keep disciplined with your reads and all of that. Then, he's a good leader of men. He's great with people and a good leader."

Nagy became Chicago's head coach in 2018 and guided the Bears to a 12-4 record last year en route to being named NFL Coach of the Year. This year has been different for Nagy, whose Bears sit 7-7 and are eliminated from postseason contention.

Nagy appreciates the fact that he can still pick up the phone and call his former mentor at any hour.

"I can't even begin to tell you how much respect I have for him as a person, as a coach, what he's taught me," Nagy said of Reid. "This year, myself, just being my second year, it's been a different year.

"There's been a lot more adversity, and so in times like that it's very easy for me to pick up the phone at midnight if I need to and call him for advice when no one's around and we can just talk. And we've done that a few times. It means a lot to me because he's very authentic."

Like a true master, Reid couldn't be happier with how his former pupil has come into his own as an NFL head coach.

"I watched him grow in the profession," Reid said. "The way he went about it, a relentless worker, all of that stuff that it takes to be a head coach, he's got. He was coach of the year last year, so he's doing pretty good."