49ers RB Christian McCaffrey out Week 18 with mild calf strain

SANTA CLARA, Calif. -- After wrapping up the NFC's top seed and earning a first-round playoff byeSunday, the San Francisco 49ers are still working through how they will deploy their starters in their regular-season finale. But one key player has already been ruled out.

Niners coach Kyle Shanahan said Monday that star running back Christian McCaffrey is dealing with a "mild" strain of his right calf and will not play in Week 18 against the Los Angeles Rams.

The good news for McCaffrey and the Niners is the injury isn't thought to be serious and McCaffrey will be ready in time to prepare for the team's first playoff game.

"We're real confident on that," Shanahan said. "It was good news. It could have been a lot worse. We're happy that they'll take care of him this week and hopefully he'll start getting to go [in practice] during our off week."

Because the Niners have already wrapped up the top seed, they essentially get an extra week to rest players who need time to get healthy. They won't play a meaningful game until the divisional round, which is slated for Jan. 20-21.

McCaffrey injured the calf early in the third quarter of Sunday's win against the Washington Commanders. He had 91 scrimmage yards before departing but said after the game he didn't believe the injury to be serious, a suspicion that was confirmed Monday.

"I think it was minor and just made the smart move, especially not knowing what was going to happen next week," McCaffrey said. "I didn't just want to go in there and make it any worse. With certain things, you can't be dumb about it."

With McCaffrey out, the Niners turned to Elijah Mitchellto carry the load. He finished with 80 yards on 17 carries with a touchdown. The 49ers also have Jordan Mason and practice squad back Tyrion Davis-Price to pick up the slack in McCaffrey's stead.

McCaffrey isn't the only Niner dealing with an injury. Cornerback Ambry Thomas played with a broken hand against Washington and will have surgery Tuesday. Shanahan said the injury is likely to keep Thomas out against the Rams, but he too should be ready to return for the postseason.

Defensive tackle Arik Armstead, who has not played since a Dec. 3 win against the Philadelphia Eagles because of a foot injury, will again sit out Sunday with the hope he can be ready for the postseason. SafetyJi'Ayir Brown(knee) is day-to-day as he continues to rehab, while receiver Jauan Jennings and offensive lineman Jaylon Moore remain in concussion protocol.

Shanahan said he has not yet made decisions on other key veterans who could use a little additional rest in the regular-season finale. Shanahan said those choices -- including on quarterback Brock Purdy-- will be made on a case-by-case basis as the week goes on.

"It's really a hard thing to do, and it's not always the best thing to do, too," Shanahan said. "We're going to take everything into account. It's still something we'll continue talking about throughout this day and tomorrow."

Shanahan added that decisions on resting starters aren't just about giving them the game off, but also about how much they will practice during the week and the trickle-down effect it can create where players who do play end up having to do more than usual and thus become more susceptible to injury.

"When you give guys a game off, practice changes too, and that can end up hurting guys a lot, just developing bad habits, and I've seen it cost a lot of teams," Shanahan said. "You rest players and that sometimes gets more players injured. So, all that stuff you've got to balance out, and that's why there's not a clear-cut decision on anything. You've got to look at each individual and each situation and play it out as the week goes."