The San Francisco 49ers are considering placing star running back Christian McCaffrey on injured reserve as he continues to battle calf and Achilles issues.

McCaffrey missed the Niners' season-opening victory over the New York Jets on Monday, having struggled with soreness in his right calf and tendinitis in his Achilles. 

On Thursday, head coach Kyle Shanahan ruled McCaffrey out of the team's Week 2 trip to face the Minnesota Vikings on Sunday, and conversations are now taking place about placing him on the injured reserve list.

If they do so, he will have to sit out at least four more games, leaving San Francisco without one of their foremost offensive weapons for a significant period of the season.

"It's something we're considering now," Shanahan told reporters. "Yesterday was his worst day. 

"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."

McCaffrey won the NFL's Offensive Player of the Year award for 2023 after helping San Francisco reach Super Bowl LVIII in Las Vegas, where they suffered a dramatic overtime defeat against the Kansas City Chiefs.

He led the NFL in rushing with 1,459 yards with 14 touchdowns in his first full season with the 49ers, having arrived in a blockbuster trade from the Carolina Panthers in 2022.

McCaffrey admitted his injury problems were causing him plenty of frustration earlier this week, telling reporters: "My mindset every week is that I'm going to play.

"There's been weeks when I'll go the full week, not even feeling good enough to do the walk-through, and then I wake up and the body's capable of a lot of cool things. 

"Mentally, I'm always prepared to play, and then I woke up and it was a group decision, a very tough one. 

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

Christian McCaffrey has reset the running back market in the NFL.

McCaffrey agreed to a two-year, $38million extension with the San Francisco 49ers on Tuesday, making him the league's highest-paid running back.

The new deal keeps McCaffrey, who turns 28 on Friday, under contract for the next four years after he signed a four-year, $64million extension in 2020 following his third season with the Carolina Panthers.

The 49ers acquired McCaffrey from the Panthers via a trade during the 2022 season, and he won the AP Offensive Player of the Year award in his first full season in San Francisco in 2023 after leading the NFL in rushing with 1,459 yards with 14 touchdowns.

He added another 564 receiving yards and seven TD receptions to help the 49ers secure the top seed in the NFC.

In 27 games with the 49ers, he has 2,205 rushing yards and 20 touchdowns to go with 1,028 receiving yards and 11 receiving TDs.

Since his 2017 rookie season, he leads all players with 10,505 scrimmage yards, while his 81 scrimmage touchdowns trail only Derrick Henry's 88.

Known for his bruising running style, McCaffrey also leads all players with 72 runs of at least 10 yards over the last two seasons.

McCaffrey attended the first day of San Francisco's mandatory mini-camp on Tuesday after skipping voluntary work-outs over the last two weeks.