Ninety minutes or so before Monday’s night’s game, there was a bit of a surprise in the Bay Area.

Christian McCaffrey, the reigning AP NFL Offensive Player of the Year, was out, and the unheralded Jordan Mason would get the start at running back for the San Francisco 49ers against the New York Jets.

Mason delivered again and again on Monday night, battering the ballyhooed Jets defense to lead the 49ers to a dominant 32-19 victory in their season opener.

“So obviously,” 49ers quarterback Brock Purdy said, “Christian is the best in the league at running back and what he does, I’m not taking anything away from that, but I think JP did a good job of coming in and filling his void and doing his job to allow other guys to get open and allow us to run our offense.”

Mason carried the ball an eye-popping 28 times for 147 yards and a 5-yard touchdown in the third quarter that pushed San Francisco’s lead to 23-7. His 28 carries were the most for any back in Kyle Shanahan’s time as 49ers head coach, per NFL Research.

“When you get him the ball, he breaks tackles, usually gets more than we block for,” Shanahan said of Mason, “when he had the good lanes he always hit them and got a bunch, but JP was awesome today.”

Boasting power and shiftiness, Mason was the catalyst for a Niners offensive attack that came back from a 7-3 first-quarter deficit with 23 straight points on five consecutive drives to put the game out of reach.

The only thing that tripped up Mason seemed to a postgame interview with ESPN’s Lisa Salters.

Asked by Salters when he found out when he was starting, Mason said Friday.

“When did I find out? Maybe, Friday, Friday night, something like that?” Mason said. “I was always prepared, I mean we went through training camp, I was RB1 in training camp, so it’s been preparing from there.”

Friday was a day before McCaffrey was listed as questionable.

Shanahan subsequently told the media he never mentioned to Mason he was starting, but underscored that he would be used in a much larger capacity than he had been before.

“I never told Jordan he was going to start,” Shanahan said. “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, I told him that he was going to have to, it wasn’t going to be like usual, it was going to be a No. 2 back that was splitting a lot of the time, but nah, he didn’t know he was doing that for sure until today.”

When it was Mason’s turn at the podium, some of the luster had clearly worn off his big game.

“That question right there is why I’m mad, that’s why I don’t really like really talking to media, because you say one thing wrong, and then you know,” Mason said when asked to clarify again when he found out when he was starting. “I don’t know, just skip that question.”

Regardless of when he was told, Mason kick-started the 49ers on their first TD drive. He carried the ball four straight times for gains of 5, 6, 9 and 18 yards before the latter was pushed back by a holding call. Two plays later, Mason ran around the right end for a 17-yard score that was also nullified by a holding call. Nonetheless, he had set a punishing tone that the Jets couldn’t stand up to.

Shanahan said McCaffrey, who was held out of each preseason game and training camp practices before returning last week, didn’t have a setback with his calf/Achilles injury, but that his calf was bothering him too much on Monday to play. The issue comes and goes, Shanahan added.

If CMC’s ailment continues to be a problem, the 49ers know they have an answer in Mason.

An undrafted free agent out of Georgia Tech, Mason made his first career start on Monday night. Prior to the game, the 25-year-old had career highs of 11 carries as a rookie in 2022 and 69 rushing yards last year in a laugher against the Dallas Cowboys. He more than doubled each of those.

Heading into Monday night, Mason was the RB1, but heading into this season he was truthfully the RB3 behind McCaffrey and Elijah Mitchell. Mitchell is out for the season.

Just how much more of Mason is seen this season is likely dependent on CMC’s health. But Mason’s confident he can take the physical toll of a starter’s reps and carries.

“It’s alright,” Mason said of how he was feeling physically. “It’s just the beginning.”

Shanahan’s found another back able to power his offense and then some. Even if it was for just one game, Mason had quite a marvelous Monday to kick off the 49ers’ season.