SANTA CLARA, Calif. — For the past two seasons, Jordan Mason has looked like a starting running back. He just couldn’t play until Monday night.

The wait may have been longer than Mason would have liked, and it came under less than ideal circumstances (he replaced the injured Christian McCaffrey), but it became clear in the season opener that Mason was more than up to the job, leading the Niners to a 32-19 home run of the New York Jets.

“I’ve been working towards this moment all training camp and I’m happy to be able to play today and show it,” said Mason.

By the time Mason was done, he had set career highs with 28 carries for 147 yards. He added a rushing score and caught a pass for another 5 yards. After the win, some wondered when the Niners knew McCaffrey would be out and Mason would be in.

In an on-field interview with ESPN’s Lisa Salters, Mason suggested he was told “maybe Friday night” that he could be considered for the starting job. Coach Kyle Shanahan indicated that no decision had been made on Friday and that nothing would be decided until Monday.

Regardless, Mason’s start came as a surprise after McCaffrey was declared inactive 90 minutes before Monday night’s game due to lingering calf and Achilles injuries. McCaffrey participated in all four of the Niners’ practices on a limited basis during the week and was officially listed as questionable on Saturday’s final injury report.

Despite all that, the expectation for McCaffrey was that he would be ready to roll. McCaffrey said Friday afternoon that he had no doubts about playing against the Jets and expressed hope that he would be in line for his usual heavy workload.

After the game, Shanahan said that “it was a little too much today” for McCaffrey to play. He noted that San Francisco “thought it would be wise to sit him out.” Shanahan added that McCaffrey had no setbacks during practice.

“It was on and off all week,” Shanahan said. “He was able to practice all week, but it still bothered him to some degree. Sometimes it would go away. Sometimes it would come back. Today it bothered him a little bit, and he didn’t feel good about it.”

McCaffrey’s injury, combined with Elijah Mitchell’s hamstring injury during training camp that sidelined him for the rest of the season, opened the door for Mason to climb the rankings, an opportunity he quickly seized with a strong training camp that likely would have propelled him past Mitchell anyway.

Mason’s performance in training camp gave the Niners confidence he was ready to step in and perform, despite never having more than 11 carries, 69 yards or 27 offensive snaps in an NFL game until Monday night.

His 28 carries were the most in a regular-season game by a Niner since Shanahan took over in 2017 and the most in a season opener for the 49ers in franchise history. His 147 were the fourth-most rushing yards in a season opener by an undrafted player of all-time and the second-most by an undrafted player on Monday Night Football.

That performance came as no surprise to those in the Niners locker room, who have watched Mason grow from an undrafted free agent in 2022 out of Georgia Tech into a player who averaged 5.6 yards per carry in his previous 33 NFL games.

“Every time he touched the ball, he looked like that, as far as I can remember,” left tackle Trent Williams said. “He looked like that tonight. … I’m sure he’s matured over the last couple of years, but it’s not like I saw a big hole and he matured. He came in pretty mature and he was always ready for his opportunity. Even as a third, fourth back, he was always ready and he came in, closed a lot of games for us and ran hard and he earned everybody’s respect.”

Many of Mason’s teammates, including receiver Deebo Samuel Sr. and fullback Kyle Juszczyk, said they were told Friday that Mason would start for McCaffrey. Shanahan said he and other coaches had been telling Mason all week that he might have an expanded role and that he needed to be ready, but it wasn’t until Monday afternoon that he told him he would definitely start.

“We found out he didn’t play today and we don’t know how he’s going to feel tomorrow or the next day,” Shanahan said. “I thought he played all week until today.”

Depending on how the rest of this week goes for McCaffrey, there may be no mystery left as the Niners prepare to travel to play the Minnesota Vikings on Sunday. San Francisco’s next two games are on artificial turf, which could hinder McCaffrey’s immediate return.

Shanahan said it’s too early to comment on that now, as McCaffrey will be monitored again later this week.

“I’ll ask the player how he feels,” Shanahan said. “If they feel good and ready to go, then they’re ready to go. If they say I feel good on grass but not on turf, that’s usually a way of saying you don’t feel good. We’ll see how he feels tomorrow. We’ll see the next day and we’ll see how he feels Sunday.”

And if the Niners need to go back to Mason, there won’t be any doubt given what he did Monday night.

“I thought he ran like he always does,” Shanahan said. “When you give him the ball, he breaks tackles and usually gets more than we block. When we had the good lanes, he always hit them and we got a lot of them, but JP was great today.”