SANTA CLARA, Calif. -- After declaring Friday that he had no doubt he would play, San Francisco 49ers running back Christian McCaffrey is out for Monday night's opener against the New York Jets.

McCaffrey has dealt with a calf and Achilles injury since the early days of training camp with coach Kyle Shanahan announcing the ailments on Aug. 6. Shanahan said then that McCaffrey would be out for a couple of weeks and not participate in the preseason. The expectation, though, was that McCaffrey would be able to get back in time for Monday's game.

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

"He looks like Christian to me," run game coordinator Chris Foerster said Friday. "You need to practice, so the guys who haven't practiced, I don't want to say there's rust, but there's always a little something that goes on just because they haven't been doing 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 at the end of last season. He suffered that one in a Week 17 win against the Washington Commanders and then did not play in a meaningless 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 the injury McCaffrey has been dealing with is similar to that one.

Before he returned to practice Tuesday, McCaffrey seemed to be moving around well during practice and said he felt "much better" when speaking to reporters on Friday.

McCaffrey is the NFL's reigning Offensive Player of the Year after posting 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.