FOXBORO — Last year, Bill Belichick had this draft crush.

He was small-school prospect, undersized and undersold. A defender without a clear position, but obvious potential. And evaluating him became more difficult as the draft drew closer.

An injury at the Senior Bowl in early February forced this Sacramento State product to sit out most of the traditional draft process. No combine invite, no lifting, no running. No chance for NFL teams to compare him side by side with other prospects.

Suddenly, his film became his resume. On that film, he hunted. Running backs, wide receivers and tight ends, none were safe from this 6-foot-3, 217-pound self-guided missile.

But how exactly, at that size and coming from the FCS level, would he translate to the NFL?

To find out, the Patriots hosted Marte Mapu for a pre-draft visit in mid-April. He impressed, and weeks later, they selected him in the third round.

Even then, Belichick didn’t know if Mapu would settle as an NFL linebacker or safety. But the kid, he figured, could play, and his new teammates were quick to learn one reason why Belichick loved Mapu.

“He’s always in the film room studying. Even day one of (spring practices) when he was a rookie,” Patriots defensive captain Deatrich Wise said. “And you can tell when coach asks question, who speaks, who corrects things. He’s always that guy.”

Fast forward, and Mapu demonstrated the power of his studying in a stunning season debut last Sunday.

After missing two months with another injury he suffered in training camp, Mapu played all 75 defensive snaps during the Pats’ loss to Miami. He aligned as a deep safety, inside linebacker, outside linebacker and even over the slot as a nickelback, finishing with seven tackles and a pass breakup. Patriots coaches also tasked him with leading the defense, relaying play-calls and making pre-snap checks.

The Dolphins tested Mapu’s leadership immediately, going up-tempo on their first drive. Except Mapu had anticipated that tempo, reasoning Miami head coach Mike McDaniel, an ex-49ers assistant, would mimic what his old boss, 49ers head coach Kyle Shanahan, had done to the Patriots the week before. Mapu relayed his gut feeling to linebacker Raekwon McMillan, who then made the tackle on Miami’s opening play, a harmless four-yard run, and scooted back to the defensive huddle.

“(He’s) a young guy, but he comes in like a vet,” McMillan said. “Things that took me two or three years to pick up on, he’s getting right now.”

For the Patriots, the timing of Mapu’s emergence couldn’t be better. Jabrill Peppers, a captain and pillar of their defense, is out indefinitely on the commissioner’s exempt list following his arrest on assault charges and drug possession. Kyle Dugger is dealing with a lingering ankle injury.

Without them, the Patriots nonetheless deployed three safeties on more than two-thirds of their defensive snaps. They trusted Mapu and rookie Dell Pettus to capably replace two of their 10 best players in a critical divisional game. For one Sunday, they delivered.

“There’s a bunch of stuff going on out there that people don’t really know,” McMillan said, “but Marte and Dell (Pettus) were on it last game.”

Pats defensive coordinator DeMarcus Covington agreed.

“I’m proud of the way (Mapu) played on Sunday,” Covington said. “I think he earned the right to do that, because I know he studied in the classroom, on the field. That’s a smart guy who prepared very, very well.”

Like the wisest Patriots defenders of the Belichick era, Mapu pulls from the team’s past to propel himself forward. Any time ex-Patriots safety Devin McCourty visits the facility, Mapu requests some 1-on-1 time with him. He wants to know how McCourty played so consistently, walking the tightrope that is the deep safety position in a single-high defense.

McCourty obliges.

“It’s cool, just to have someone that’s been so great at what they do,” Mapu said. “And everybody has their own style, but to hear from him and how he played in this system, it’s so specific that it really helps.”

Mapu, other team sources says, is interested in little outside of football. He keeps a low profile. He’s straightforward. Asked about recovering from such a sudden workload last week, Mapu didn’t mince words.

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“Move as slow as possible,” he said. “Rest as much as possible.”

Mapu also explained eventually last week’s game reached a point of stasis. The Dolphins wanted to run the ball, and the Patriots were intent on playing three-safety personnel to withstand their rushing attack, while keeping enough speed on the field to defend Tyreek Hill and Jaylen Waddle. The game slowed.

Eventually Miami wore the Pats down, breaking through for a game-winning three-yard touchdown run in the fourth quarter. Yet Mapu never left the field, solely focused on fulfilling his assignment down after down; wherever and however he was asked to play.

“I didn’t want to do too much or try to identify too many (of Miami’s) plays,” he admitted. “but that’s what growth is for.”

Growth, and the foreseeable future as the new glue of the Patriots defense

Play-calling lessons

Halloween is almost three weeks away, but Alex Van Pelt feels haunted already.

The Patriots’ 54-year-old offensive coordinator volunteered Thursday he still regrets play-calls from the team’s loss at the Jets in Week 3. In the days after that game, Van Pelt said he got too pass-happy.

On Thursday, he reviewed his first five games as a play-caller.

“I feel like I’ve put us in good situations, I’ve had some bad calls,” he said. “Obviously the Jets game haunts me a little bit so in that game, but we all collectively just have to better. We have to do better in situations, situations like that. At the same time, we all just need to step up our game across the board. That goes for everyone, coaches, players.”

Covington has led another struggling unit, which ranks 28th by DVOA and 23rd by EPA/play. Covington has rebounded since Seattle’s Geno Smith and Jets quarterback Aaron Rodgers torched his blitz package in back-to-back weeks. But managing a banged-up roster has proved to be his greatest challenge.

“It’s just, for me, to continue to evolve as the season goes on because it seems like every week we have, whether it’s a player that goes out, a different unit that needs to step up, I need to help the players play in the right way, based off who’s available for the game,” Covington said. “We haven’t had a consistent group this season. It’s always been somebody different, whether, like last week we had both dugout, we had family out and that, like every single week.

“So just trying to make sure that we’re all playing together as a unit, and then making sure that we tailor their game plan for who we have out there on the field. So I think that’s, that’s the biggest thing for me as a play-caller.”

Coaching connections

When the Patriots’ defensive braintrust stares across the sideline Sunday, they will spot a familiar face.

Texans head coach DeMeco Ryans is a fellow Alabaman that Covington followed as a high school and college player, also from the greater Birmingham area. Ryans played at the University of Alabama, where he became a unanimous All-American and eventually left to play 10 years in the NFL. Since then, he’s enjoyed a meteoric rise through the NFL coaching ranks, making the playoffs last season as a first-year head coach after two seasons as the 49ers defensive coordinator.

“I’m proud of him, the success you see,” Covington said. “You know, a Black head coach in the National Football League, and just the success he’s had and what he’s done with that team. I’m proud of him, and I look up to what he’s doing over there … (They) fly around, (he) gets those guys playing hard.”

Shortly after Ryans left Alabama, Jerod Mayo was coming up as a new star linebacker for the University of Tennessee. Mayo, 38, admitted this week he used to chase Ryans’ tackle records in the SEC.

“I would say the history between DeMeco and myself goes back a long way, and he doesn’t even know it. He was one of those guys I looked up to. In college, you try to chase those tackle numbers. I would put Patrick Willis in that same realm, as far as me as a young guy trying to chase those guys as far as stats are concerned. He’s done a fantastic job. … I look up to DeMeco and that entire organization, and hopefully we can replicate some of that stuff here in the near term and in the future,” he said.

Quote of the Week

“He played his balls off last week.” — Patriots defensive line coach Jerry Montgomery on defensive tackle Daniel Ekuale