FOXBORO — Inside a dark room in the dark days of his playing career, Jerod Mayo learned what every Patriots defender to sit with Steve Belichick has discovered since.

He’s smart. He’s detailed. And just like his father: X’s and O’s are like ABCs.

Back then, Mayo was an injured linebacker whose path to significance on Sundays ran through the film room, where he’d sit for hours with the eldest son of Patriots head coach Bill Belichick. Steve Belichick was a coaching assistant whose job was to log long hours in such rooms day after day. This season, they’re again working together; now under a soft spotlight.

In the preseason, Mayo, the Pats’ first-year inside linebackers coach, called defensive plays. It brought significant attention to his ascent as the fastest-rising assistant ever under Bill Belichick. But around the time the games started to count, Mayo passed the play sheet to his old film room partner.

Patriots secondary coach Steve Belichick has been calling defensive plays for weeks, multiple players confirmed to the Herald. Bill Belichick remains ever observant on the sideline, jotting notes and getting a feel for the game. Whenever he deems necessary, the headman will turn from the field and address his defense with an adjustment.

The defense is still his, after all. Everyone will attest. The play calls, though, those are coming from a different headset.

“We’ve started to gel with (Steve Belichick), knowing what he’s going to call and the way he thinks through the game,” Patriots safety Terrence Brooks said. “I remember before one of his first games calling it, he was a little nervous, and I had to shake him up a little bit and pep talk him up and tell him that he’s one of the best.

“He was just sitting there, and I was like, ‘C’mon, get it together. Let’s go! You’re one of the best. This is nothing’. And he popped up.”

Brooks’ pep talk was his way of returning the favor. Belichick, the safety said, inspires him before every game with a similar speech. It’s partly why Brooks is so endeared to his position coach, with whom he said he wouldn’t mind spending the rest of his career.

“Man, I love that dude. I really do. That’s my guy,” Brooks said. “He’s just a great coach. He understands his players, he goes out there and he wants you to do right. He’s pulling for you. You always come across some coaches who just wanna yell, but he makes you learn, he teaches you things and expects a lot out of his DBs.”

As the most experienced assistant left on the defensive staff this offseason, Steve Belichick naturally engendered trust from Patriots players. He knew the system, he knew them. Even players at other positions felt a connection to him.

Linebacker Elandon Roberts has worked with Steve Belichick since coming into the league in 2016, the first year Belichick served as the team’s safeties coach. So when former defensive play-caller Brian Flores left for Miami, at no point has he been worried about the transition.

“(Flores) was my linebackers coach, so once he became the play-caller, it was easy to trust what he was saying. And I’ve been here with Steve the whole way. So it’s easy to trust him,” Roberts said before later adding: on the “Coaching side of it, he gets the highest praise from me.”

Belichick dodges questions about what his exact duties are on game days, even once flatly denying he knows what it’s like to prepare for play-calling. He does admit that his role has changed, and he is enjoying it.

“It’s what you work for — to go out and compete on Sundays,” Belichick said. “We’ve got a great group of defensive players. They play hard. It’s been a lot of fun working with Jerod and everybody on staff.”

Mayo said Belichick continues to “drop nuggets” of information on Patriots players and coaches in meetings, much like he did in their original film sessions.

“He’s also one of those guys, like his dad, appreciates the history of the game,” Mayo said, “appreciates the X’s and O’s and how the game’s evolved.”

At this time, the Patriots are the ones making history.

They rank first in the NFL in points allowed per game and DVOA (defense-adjusted value over average). They’ve allowed fewer points through eight weeks than any of the all-time defenses they’ve been compared to — the ’70s Steelers, ’85 Bears or 2000 Ravens. They’re on pace to become the greatest defense of all time, albeit with several significant challenges lying ahead.

Soon enough, the history Steve Belichick can appreciate next may be some of his own making.