SANTA CLARA, Calif. — After stating Friday that he had no doubts he would play, San Francisco 49ers running back Christian McCaffrey will not attend Monday night’s season opener against the New York Jets.

McCaffrey has been battling a calf and Achilles injury since the early days of training camp, with coach Kyle Shanahan announcing the ailments on August 6. Shanahan said at the time that McCaffrey would be out for a few weeks and would not participate in the preseason. However, McCaffrey was expected to return in time for Monday’s game.

On Friday, McCaffrey even said he hoped to get his usual heavy workload. And general manager John Lynch told KNBR radio Friday morning that McCaffrey would be “in and ready to roll.” But even after participating in a limited capacity in four practices this week, McCaffrey was ruled out about 90 minutes before the game against the Jets as one of their six inactive players (plus the emergency third quarterback). The Niners listed McCaffrey as questionable against the Jets even after teammates and coaches expressed optimism about his chances to play and contribute anything close to his usual level.

“He looks like Christian to me,” run game coordinator Chris Foerster said Friday. “You have to practice, so the guys that haven’t practiced, I don’t want to say there’s rust, but there’s always something that happens, just because they don’t do it every day. There’s just that grind of every day, day after day. But he’s looked good.”

McCaffrey dealt with a similar injury late last season. He suffered it in a Week 17 win over the Washington Commanders and then sat out an inconsequential Week 18 game against the Los Angeles Rams. He returned for the NFC divisional round against the Green Bay Packers about three weeks after that initial injury. Shanahan said Saturday that the injury McCaffrey is dealing with is similar to that one.

Before returning to practice on Tuesday, McCaffrey appeared to be moving fine during practice, saying he felt “much better” when he spoke to reporters on Friday.

McCaffrey is the NFL’s reigning Offensive Player of the Year after recording 1,459 rushing yards, 564 receiving yards and 21 total touchdowns in 2023.

Without McCaffrey, Jordan Mason is expected to start at running back. This would be Mason’s first career start after averaging 5.6 yards per carry in 33 career games. The former undrafted free agent out of Georgia Tech has never had more than 69 rushing yards, 11 carries or 27 snaps in a game.

Veteran Patrick Taylor Jr. and rookie Isaac Guerendo are the team’s other active running backs Monday night.

Elijah Mitchell, the team’s reserve running back last season, is on the injured list and will be out for the season with a hamstring injury.