SANTA CLARA, Calif. -- When it comes to San Francisco 49ers running back Christian McCaffrey's availability, no guesswork is needed this week.

Niners coach Kyle Shanahan on Friday officially ruled McCaffrey out for Sunday's game at the Minnesota Vikings after McCaffrey didn't feel right with limited participation in Thursday's practice. Placing McCaffrey on injured reserve, which would mean missing at least four more games, is now a part of the conversation, according to Shanahan.

"It's something we're considering now," Shanahan said. "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, of course, has been dealing with a calf injury and Achilles tendinitis that made him a surprise pregame scratch before Monday night's opener against the New York Jets.

Like last week, McCaffrey was a limited participant in practice Wednesday and Thursday, but signs of his potential deactivation were clear when he was not on the field during the early portion of Friday's practice that was open to media.

If the Niners place McCaffrey on injured reserve before Sunday's game, he would be eligible to return for the Oct. 10 game against the Seattle Seahawks on "Thursday Night Football." Shanahan added that as of now, McCaffrey does not require any sort of procedure to repair the injury.

McCaffrey spoke to reporters earlier this week and clarified why he was so adamant before the Jets game that he was going to play. McCaffrey explained that whenever he is dealing with an injury, he mentally tells himself he is going to be ready to go even if that ultimately doesn't turn out to be the case.

"My mindset every week is I'm going to play," McCaffrey said Wednesday. "There's been weeks where 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."

McCaffrey's inactive status Monday night came as a surprise, but a league spokesperson told ESPN's Adam Schefter that the NFL looked into the matter and "found no evidence of a violation of the league's Injury Report Policy."

The Niners' hope is that more rest for McCaffrey will allow him to put the tendinitis in the past. Shanahan has said that the hard part is that the issue with the Achilles comes and goes, which has made it difficult to evaluate it on anything other than a day-to-day basis.

Shanahan was asked Wednesday whether McCaffrey could be a candidate for injured reserve. He said at the time that had not been a consideration, but both he and general manager John Lynch struck a different tone Friday after this latest setback.

"If that's the right thing, it's what we'll do," Lynch told KNBR Radio in San Francisco. "All options, as Kyle has said, are on the table. We'll do what's right for Christian and what's right for our team."

This week, that means a second consecutive start for third year-running back Jordan Mason after his breakout performance Monday night. In his first career start, Mason rushed 28 times for 147 yards and a touchdown with a catch for 5 yards.

It was the most carries in a regular-season game by a Niner since Shanahan took over in 2017, and the most in a 49ers season opener in franchise history.

That performance undoubtedly buoyed San Francisco's confidence in Mason to make another start as the team travels to Minnesota, a place where the Niners have lost seven consecutive games.

"I keep saying, this guy just keeps getting better and better and better," run game coordinator Chris Foerster said. "JP always worked hard, but there was always that little bit of a trust factor there. Now he's all in with (running backs coach) Bobby (Turner). He's all in with the offense and he has just flourished. He's a tough, hard running guy."

With McCaffrey out and Mason starting, the Niners' backup options will again be receiver Deebo Samuel, rookie Isaac Guerendo and fourth-year veteran Patrick Taylor Jr. Neither Guerendo nor Taylor got a carry behind Mason against the Jets, though, as Samuel was the only other Niner to take a handoff, rushing eight times for 23 yards and a touchdown.

"It's different to be down to your third guy already," Shanahan said. "I've never really looked at Elijah (Mitchell) or JP (Mason) as two or threes. I think those guys are capable of being ones as Elijah has shown in the past and so did JP. The two new guys when their (opportunity) comes, I believe they're ready."

In other Niners injury news, safety Talanoa Hufanga (right knee) is doubtful for Sunday and linebacker Dee Winters (ankle) is listed as questionable.