MINNEAPOLIS -- The San Francisco 49ers are placing star running back Christian McCaffrey on injured reserve with calf soreness and Achilles tendinitis, sources told ESPN's Adam Schefter on Saturday.

McCaffrey must miss at least the next four games -- including Sunday's matchup with the Minnesota Vikings -- and is eligible to return for an Oct. 10 meeting with the Seattle Seahawks on "Thursday Night Football," though that's not a guarantee.

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

"Yesterday was his worst day," Shanahan said. "It's on and off, but with yesterday having the most pain. It's something we're going to be discussing together in the next 24 hours."

Those discussions ultimately led to Saturday's decision to shut McCaffrey down in hopes that the continued flare-ups in the Achilles will settle down. Shanahan has said the hardest part of McCaffrey's injury has been its unpredictability, with the running back feeling good and practicing some days and then feeling pain on others.

Landing on injured reserve is undoubtedly a disappointment for McCaffrey, the NFL's reigning Offensive Player of the Year. As recently as Wednesday, he said he planned to play against the Vikings, explaining that his mindset every week is that he is going to be ready to go even if he is dealing with an injury.

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

This IR stint is McCaffrey's first since joining the 49ers via trade in October 2022. Including the postseason, McCaffrey had missed only one of 33 possible games over the previous two seasons, with the lone miss being a meaningless Week 18 game against the Los Angeles Rams in which McCaffrey was dealing with a calf injury but both teams were resting starters anyway.

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

With McCaffrey out, Jordan Mason will handle the primary running back duties after making the first start of his career 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 main 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.