SANTA CLARA – Running back Christian McCaffrey’s calf and Achilles issues made him a game-time scratch before the 49ers’ season opener Monday night, as first reported by this news organization.

McCaffrey, last season’s NFL rushing champion and AP Offensive Player of the Year, was listed as questionable, meaning he had a 50/50 chance of playing. He insisted Friday he had no doubt he would play.

Jordan Mason is in line to make his first career start when the 49ers kick off against the New York Jets, on a night in which the 49ers’ all-time leading rusher Frank Gore will be honored at halftime for his induction to the franchise’s hall of fame.

Other running backs available are Patrick Taylor Jr. and rookie Isaac Guerendo.

McCaffrey missed only one previous start since joining the 49ers in a 2022 midseason trade, and that lone absence was January’s regular-season finale against the Rams in which the 49ers rested most starters because the No. 1 seed was already secured for the NFC playoffs.

Mason had a career-high 27 snaps in that game, carrying six times for 36 yards. The most carries Mason has had in a game is 11 (for 56 yards) as a rookie in a 35-7 win over Tampa Bay on Dec. 11, 2022. Four days later, Mason had his biggest yardage game with 64 yards on four carries in a 21-13 win over Seattle.

It will be the first game 49ers quarterback Brock Purdy has played without McCaffrey in the lineup. Mason has 83 career carries for 464 yards, averaging 5.6 yards per carry with four touchdowns in 33 games.