SANTA CLARA >> A 49ers running back set the tone for a San Francisco offense that put up 32 points in a season-opening rout of the New York Jets.

But it wasn’t Christian McCaffrey rolling up 147 rushing yards and a touchdown on 28 carries at Levi’s Stadium. The Madden 25 cover athlete was a late scratch with a calf injury.

In Aaron Rodgers’ long-awaited return, it was Jordan “JP” Mason who wowed in front of a national television audience on Monday Night Football in San Francisco’s 32-19 victory.

With McCaffrey out for the entire preseason, Mason said the increased workload, along with a singular focus on fitness, helped him take advantage of the opportunity.

Mason told ESPN that he knew he’d start days before kickoff.

“When did I find out? Maybe Friday night,” Mason said on the broadcast after putting up career highs in carries and yards. “I was always prepared.”

Mason’s coach was supremely confident in him, but disputed Mason’s timeline of events.

“I never told Jordan he was gonna start,” Shanahan told reporters a few minutes later at the team’s media availability. “Told him he had to be ready a bunch, but it might have been (running backs coach) Bobby (Turner) or somebody trying to pump him up. But I knew he was going to have to play a lot.”

Mason later expressed regret over his answer in his terse appearance at the team’s postgame news conference.

“That question right there is why I’m mad,” Mason said at the podium. “That’s why I don’t like talking to media because you say one thing wrong and then, you know. I don’t know — just skip that question.”

Regardless of which one was telling the truth, Shanahan was correct about Mason’s workload.

Mason was the man in the backfield, and he played the majority of the snaps.

The third-year pro out of Georgia Tech by way of Gallatin, Tennessee didn’t try to fill McCaffrey’s MVP-sized shoes by attempting to break runs to the outside the way the 2023 Offensive Player of the Year can.

His longest rush was only 24 yards, the majority of Mason’s totes being no-frills but effective. The surprise starter found the hole, ran through it and fell forward upon contact.

“It was no surprise for me, the way he’s been working in camp,” Samuel said. “How hard he runs, he’s going to break tackles. He came in and had a great day.”

The 223-pounder’s lone touchdown was a perfect example of that.

Mason’s second-quarter score was, yes, a magnificent display of blocking.

Center Jake Brendel and left guard Aaron Banks washed their defenders out of the way, and every receiver and tight end to that side did their part to block.

But Mason also displayed phenomenal vision — cutting back into space — and then featured enough balance to fight through multiple tacklers before falling into the end zone.

This was nothing his teammates hadn’t seen before.

“Even in the offseason, I saw him with a different mindset that he was going to be ready for his moment when it did come,” Fred Warner said.

Brock Purdy remembered when Mason was — like himself — an unheralded rookie in the 49ers training camp back in 2022. He went undrafted out of Georgia Tech but carved out a role on the team for the past two seasons.

“Obviously Christian is the best in the league at running back and what he does — not taking anything away from him — but I think JP did a good job of coming and and filling his void and doing his job to allow other guys to get open and allow us to run our offense,” Purdy said.

The former Yellowjacket had only shown brief flashes of this ability in his previous two years as a rarely used backup for McCaffrey.

In the 18 games in which he received a carry over the past couple of seasons, Mason’s career-high in carries was 11 and his best day in terms of yardage was only 69.

Having proven that he can handle a far larger workload, Mason doesn’t want to be a flash in the pan.

“It’s just the beginning,” he proclaimed at the podium.