EAST RUTHERFORD, N.J. — Sam Darnold entered the bowels of MetLife Stadium at 9:51 a.m. Sunday, coffee in hand and mentor Josh McCown by his side. The New York Jets’ rookie quarterback exuded quiet confidence, flashing an easy smile as he walked toward the home team’s locker room. His mission was simple: Ride the momentum of their Motor City victory and dispose of the Miami Dolphins in front of the ever-faithful, yet oh-so-fatalistic home crowd.

This was Darnold’s moment, his chance to live up to the buzz that had been building throughout the Tri-State area all week, culminating in hour-long traffic jams en route to the parking lots and tailgating. But nothing is ever simple for NFL rookies, particularly quarterbacks. And seven hours after Darnold arrived in the building, he was lamenting the mistakes that turned a winnable game into a head-scratching defeat.

The 21-year-old threw his first pair of interceptions and suffered his first professional defeat against the AFC East rival Miami Dolphins, who exited The Meadowlands with a 20-12 victory and a 2-0 start. They “rattled” Darnold early “and didn’t let up,” noted defensive end Robert Quinn, who accounted for one of three Dolphins sacks Sunday afternoon. But Darnold’s stat line — 25-for-41, 334 yards, one touchdown and a 74.6 quarterback rating — told a familiar story.

The hype that built up following Darnold’s rookie debut in the Jets’ 48-17 manhandling of the Lions dissipated with each stalled drive and every errant throw made against the Dolphins. Unlike last week, when Darnold rebounded from throwing a pick-six on his very first snap, there was no comeback, despite it being only a one-possession game with 5:56 remaining.

“It’s not a good feeling,” said Darnold, whose 28-yard touchdown pass to running back Bilal Powell pulled the Jets to within 20-6 before kicker Jason Myers missed the extra point in the third quarter.

This week, there was no Jets victory, no celebratory hugs and high-fives in the locker room. Instead, only soothing words whispered in the ear of a crestfallen quarterback. And therein lies the potential key to Darnold’s NFL success: The ever-present, never-wavering support system around him.

Within minutes of concluding his postgame news conference, Darnold was back in the locker room listening intently to words of encouragement from his teammates. He leaned forward at his locker stall while veteran wide receiver Jermaine Kearse stood to his left and spoke in hushed whispers before patting the rookie on the shoulder and walking into the trainers room. The players to Darnold’s immediate right — Powell and fellow running back Isaiah Crowell — took their turn, leaning over to offer their takes, their perspective, their support.

“You can’t win all of them,” Powell said, divulging his message to his quarterback. “ … When you have a guy like Josh McCown over there, there isn’t a need for me to say too much to him. [Darnold] shook it off. We came back, we drove down the field. We didn’t get the ‘W, ’ of course. But we’ll shake it off.”

By 4:30 p.m. — more than seven hours after their arrival — Darnold was next to McCown again.

The two quarterbacks, who share more of a brotherly bond than anything, sat side by side, gesturing different routes with their hands as they dissected mistimed plays and missed opportunities.

There were chances. Man, were there chances.

Case in point: When Darnold admittedly didn’t see a wide-open Quincy Enunwa in the end zone and threw the ball away near the end of the first half. And on the very next play, with no timeouts remaining and his team trailing 20-0, Darnold completed a pass to tight end Chris Herndon, who was stopped at the 1-yard line as time expired.

Receiver Terrelle Pryor publicly shouldered the blame for Darnold’s second interception, which was picked off in the end zone by cornerback Xavien Howard immediately after a Jets defender forced and recovered a fumble. “I said it to him after the game, ‘I hope you forgive me. I’ll make sure I’m there next time,” Pryor said, telling reporters that he apologized to the rookie for running a sloppy route.

Poor route-running, penalties and mental mistakes on both sides of the ball culminated to produce a seemingly avoidable loss. And now, the Jets have only a handful of days to erase it from their minds and refocus for their prime time road matchup with the still-winless Cleveland Browns on “Thursday Night Football.”

“Every week is a different challenge,” Darnold said. “We’re going to get after it this week and get ready for Cleveland … and hopefully we’ll get a win.”

Three games in 11 days is enough to make a rookie’s head spin. But those around him are determined not to see him falter.

At 4:50 p.m., he exited the locker room and locked eyes with his family. Darnold’s father, Mike, pointed at his son and smiled, while his mother, Chris, gave her son a sympathetic smile. Just like last week in Detroit, the Darnolds waited, alongside Jets general manager Mike Maccagnan, for their son and McCown to emerge from the locker room. And while there was no victory to celebrate this time, there were plenty of shared hugs between them.

After words are exchanged between the Darnold clan, Mike reminds his son to grab some catered food before he leaves the stadium. The rookie quarterback initially declines, but soon relents. Father, apparently, knows best.

Darnold finally heads toward the exit, food in hand, flanked by his family. Another game under his belt. Another day in the life of a rookie.

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