Running back Christian McCaffrey is out for the game against the Vikings as the 49ers consider placing him on the interim list

SANTA CLARA, Calif. — There's no guessing involved this week as far as San Francisco 49ers running back Christian McCaffrey's availability is concerned.

Niners coach Kyle Shanahan ruled McCaffrey out of Sunday's game at the Minnesota Vikings on Friday after the running back was uncomfortable after limited participation in Thursday's practice. Placing McCaffrey on the injured list, which would mean he would have to miss at least four games, is now part of the discussion, Shanahan said.

“That's something we're thinking about now,” Shanahan said. “Yesterday was his worst day. It goes up and down, but yesterday was the most painful day. That's something we'll discuss together over the next 24 hours.”

McCaffrey is battling a right calf injury and Achilles tendonitis, which is why he was surprisingly not used before Monday night's opening game against the New York Jets.

Like last week, McCaffrey participated in limited practice on Wednesday and Thursday, but signs of his potential exhaustion were clear when he was absent from the field at the start of practice Friday, which was open to reporters.

If the Niners place McCaffrey on the injured list before Sunday's game, he would be eligible to play on Thursday Night Football against the Seattle Seahawks on Oct. 10.

Shanahan added that McCaffrey does not require any surgery to repair his injury at this time.

McCaffrey spoke to reporters earlier this week and explained why he was so adamant about wanting to play before the Jets game. McCaffrey explained that whenever he's battling an injury, he mentally tells himself he'll be ready to play, even if that ultimately turns out not to be the case.

“Every week I think I'm going to play,” McCaffrey said Wednesday. “There have been weeks where I've gone the whole week and not even felt good enough to do the practice round, and then I'd wake up and my body was capable of a lot of cool things. Mentally I'm always ready to play, and then I'd wake up and it was a group decision, a very tough one. I hate not playing, but I see it as a move and something that hopefully will benefit the rest of the season.”

McCaffrey's inactivity status on Monday night came as a surprise, but a league spokesman told ESPN's Adam Schefter that the NFL investigated the matter and “found no evidence of a violation of the league's injury reporting policies.”

The Niners hope that with more rest, McCaffrey can put his tendinitis behind him. Shanahan said the difficult part is that the Achilles problem comes and goes, making it difficult to evaluate other than on a day-to-day basis.

Shanahan was asked Wednesday if McCaffrey could be a candidate for injured reserve. He said at the time that it hadn't 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 to do, we'll do it,” Lynch told KNBR radio in San Francisco. “All options, like Kyle said, are on the table. We'll do what's right for Christian and our team.”

This week, that means three-year running back Jordan Mason will make his second consecutive start after his standout performance on Monday night. In his first career start, Mason ran 28 times for 147 yards and scored a touchdown with a 5-yard catch.

It was the most turnovers by a Niners player in a regular season game since Shanahan took over in 2017 and the most in a 49ers season opener in franchise history.

That performance has undoubtedly boosted San Francisco's confidence in Mason as the team travels to Minnesota, where the Niners have lost seven games in a row and he will once again start.

“I keep saying, this guy just keeps getting better and better and better,” said run game coordinator Chris Foerster. “JP has always worked hard, but there was always that little confidence factor. Now he's fully in with [running backs coach] Bobby [Turner]. He's totally on the offensive and is really blossoming. He's a tough guy who loves to run.”

With McCaffrey out and Mason in the starting lineup, the Niners' backup options once again include wide receiver Deebo Samuel, rookie Isaac Guerendo and fourth-year veteran Patrick Taylor Jr. Neither Guerendo nor Taylor had a carry behind Mason against the Jets, however, as Samuel was the only other Niners player to get a handoff, running eight times for 23 yards and a touchdown.

“It's different when you're on the third guy,” Shanahan said. “I never really looked at Elijah. [Mitchell] or JP [Mason] than two or three. I think these guys are capable of being ones, as Elijah has shown in the past, and JP as well. The two new guys, as their [opportunity] comes, I think they are ready.”

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