MINNEAPOLIS — The San Francisco 49ers have placed star forward Christian McCaffrey on the injured list with calf soreness and Achilles tendonitis, the team announced Saturday.

McCaffrey will miss at least the next four games, including Sunday’s game against the Minnesota Vikings, and will be eligible to play again on October 10 against the Seattle Seahawks on ‘Thursday Night Football’, but that is not yet certain.

On Friday, San Francisco coach Kyle Shanahan acknowledged for the first time since announcing McCaffrey’s injury on Aug. 6 that injured reserve was a real possibility after McCaffrey’s Achilles did not respond well to limited participation in practice on Thursday.

“Yesterday was his worst day,” Shanahan said. “It comes and goes, but yesterday was the most painful day for him. It’s something we’re going to talk about together over the next 24 hours.”

Those discussions ultimately led to Saturday’s decision to sit McCaffrey out in hopes that the ongoing Achilles flare-ups will subside. Shanahan has said the hardest part about McCaffrey’s injury is its unpredictability, with the running back feeling fine and working out some days and being sore others.

That McCaffrey, the NFL’s reigning Offensive Player of the Year, is going on the injured list is undoubtedly a disappointment. As recently as Wednesday, he said he planned to play against the Vikings, explaining that he’s focused on being ready to go every week, even if he’s dealing with an injury.

“I hate not playing,” McCaffrey said Wednesday. “But I look at it as a chess move and something that you can hopefully benefit from the rest of the season.”

This is McCaffrey’s first IR stint since joining the 49ers via trade in October 2022. Including the postseason, McCaffrey had missed just one of a possible 33 games over the previous two seasons. The only one he missed was a meaningless Week 18 game against the Los Angeles Rams, where McCaffrey was dealing with a calf injury. Both teams rested their starters anyway.

Before arriving in San Francisco, McCaffrey struggled with injuries as a member of the Carolina Panthers. In 2020 and 2021, McCaffrey played a combined 10 games, missing games with a high right ankle sprain, a sprained AC joint in his shoulder, a strained glute and hamstring, and a sprained left ankle.

With McCaffrey out, Jordan Mason will assume the primary running back duties after making his first career start last week against the New York Jets. Mason finished with 28 carries for 147 yards, both career highs, and scored a touchdown. It was the most carries in a regular-season game by a Niners player since Shanahan took over in 2017 and the most in a 49ers season opener in franchise history.

Against the Jets, the Niners used wide receiver Deebo Samuel Sr. as Mason’s primary backup. He had eight carries for 23 yards and a touchdown. Other running back options for the 49ers include rookie Isaac Guerendo and fourth-year veteran Patrick Taylor Jr. on the active roster and fifth-year veteran Ke’Shawn Vaughn on the practice squad.

The 49ers have not yet filled McCaffrey’s spot on the 53-man roster.