Mason carried the ball 28 times for 147 yards and a 5-yard touchdown in the third quarter to extend San Francisco’s lead to 23-7. According to NFL Research, his 28 carries were the most of any player during Kyle Shanahan’s tenure as head coach of the 49ers.

“When you give him the ball, he breaks tackles, and usually he makes more tackles than we block,” Shanahan said of Mason, “when he had the good lanes, he always hit them and caught a lot of them, but JP was great today.”

With his strength and agility, Mason was the catalyst for a Niners offense that rallied from a 7-3 first-quarter deficit with 23 straight points on five consecutive drives to decisively decide the game.

The only thing Mason struggled with was a post-game interview with ESPN’s Lisa Salters.

When asked by Salters when he would find out he would be allowed to start, Mason replied on 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 from there it’s preparation.”

Friday was the day before McCaffrey was deemed questionable.

Shanahan later told the media that he never told Mason he would start, but he did emphasize that he would be used in a much larger role than before.

“I never told Jordan he was going to start,” Shanahan said. “I told him he had to be ready a lot, but it could have been (running backs coach) Bobby (Turner) or somebody trying to get him fired up. But I knew he was going to have to play a lot, I told him he was going to have to do that, it wouldn’t be like he usually does, it would be a No. 2 back splitting a lot of the time, but no, he didn’t know for sure he was going to do that until today.”

When Mason took the stage, the shine had clearly faded from his playing.

“That question is exactly why I’m angry, that’s why I don’t like to talk to the media, because you say one thing wrong, and then you know,” Mason said when asked to clarify again when he figured out when he was starting. “I don’t know, just skip that question.”

Regardless of when he was told, Mason kick-started the 49ers’ first TD drive. He carried the ball four times in a row for gains of 5, 6, 9 and 18 yards before the last one was pushed back on a holding call. Two plays later, Mason scampered around the right sideline for a 17-yard score that was also negated by a holding call. Still, he had set a strong tone that the Jets could not match.

Shanahan said McCaffrey, who missed every preseason game and training camp practice before returning last week, had no setbacks with his calf/Achilles injury, but his calf bothered him too much to play Monday. The issue comes and goes, Shanahan added.

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

Mason, an undrafted free agent out of Georgia Tech, made his first career start Monday night. Before 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 doubled each of those marks.

On Monday night, Mason was the RB1, but this season he was actually the RB3 behind McCaffrey and Elijah Mitchell. Mitchell is out for the season.

How much more Mason sees this season likely depends on CMC’s health, but Mason is confident he can handle the physical toll of a starter’s reps and carries.

“It’s okay,” Mason said of how he felt physically. “It’s just the beginning.”

Shanahan has found another back who can power his offense and more. Even if it was just for one game, Mason had a great Monday to start the 49ers’ season.