(Photo credit: Sergio Estrada-Imagn Images)

The San Francisco 49ers practiced Tuesday without two key pieces of their offense, running back Christian McCaffrey and left tackle Trent Williams.

They were not mere veterans' rest days. McCaffrey appeared on the injury list with a new back issue, and Williams was listed with the right hamstring injury sustained Sunday night in San Francisco's 42-38 win over the Chicago Bears.

The NFC West title and No. 1 playoff seed in the conference are on the line when the 49ers host the Seattle Seahawks on Saturday, and it's up in the air whether either former All-Pro player will miss the game.

Niners coach Kyle Shanahan called McCaffrey's ailment 'back stiffness,' and the running back insisted to reporters Tuesday it was not serious. As for Williams, he left the Bears game after hurting his hamstring on the first play of the night.

'Not sure how long (he'll be out),' Shanahan said. 'He's got a shot for this week.'

McCaffrey has battled injuries throughout his career but has started all 16 games for San Francisco thus far this season. He has compiled a career-high 303 carries for 1,179 rushing yards and 10 touchdowns while adding 96 receptions for 890 yards and seven scores.

The 49ers listed tight end George Kittle (ankle) as a limited participant in Tuesday's practice. Kittle sat out the Bears game, and his return for Week 18 is no guarantee.

'I wouldn't assume that' Kittle will play, Shanahan said. 'We're barely moving today. We have a short week, so we won't be doing much all week.'

Wide receiver Ricky Pearsall (knee/ankle) and defensive end Yetur Gross-Matos (knee) were the other notable 49ers who were limited on Tuesday. Defensive lineman Keion White (groin) and fullback Kyle Juszczyk (hip) sat out.

The Seahawks held a walkthrough on Tuesday and listed starting left tackle Charles Cross (hamstring) and his backup Josh Jones (ankle/knee) as DNPs on their estimated report. Wide receiver/return specialist Rashid Shaheed (concussion) and safety Coby Bryant (knee) were limited.

--Field Level Media