SANTA CLARA >> Running back Christian McCaffrey’s calf and Achilles issues made him a game-time scratch before the 49ers’ season opener Monday night.

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

Jordan Mason made his first career NFL start when the 49ers kicked off against the New York Jets on a night in which the 49ers’ all-time leading rusher Frank Gore was honored at halftime for his induction to the franchise’s hall of fame.

Other running backs available were Patrick Taylor Jr. and rookie Isaac Guerendo. Deebo Samuel was also expected to pick up the slack, having averaged over three carries per game since his 2021 All-Pro season.

McCaffrey missed only one previous start since joining the 49ers in a 2022 midseason trade, and that lone absence came during the 2023 regular-season finale against the Rams on Jan. 7. That game, 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.

Monday’s nationally televised game marked the first time 49ers quarterback Brock Purdy had 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.

The 49ers also deactivated safety Talanoa Hufanga, who was doubtful to delay his return from last November’s knee injury. Defensive end Yetur Gross-Matos and linebacker Dee Winters were ruled out Saturday, and joining them on the inactive list along with McCaffrey and Hufanga were cornerback Darrell Luter Jr., offensive lineman Ben Bartch and quarterback Joshua Dobbs, who will dress as the emergency option if Purdy and Brandon Allen get hurt.

In other pregame transactions, the 49ers formally activated left tackle Trent Williams off the reserve/did-not-report list and released offensive tackle Brandon Parker. Defensive end Sam Okuayinonu and safety Tracy Walker III were summoned from the practice squad.

Inactive for the Jets: quarterback Max Mitchell, running back Israel Abanikanda, tackle Carter Warren, tight end Brenden Bates, cornerback Jarrick Bernard-Converse and defensive linemen Eric Watts and Leonard Taylor.