The San Francisco 49ers have spent most of this season working around injuries, and at 7-1, they’ve done it better than anyone could have expected.

The latest one is different, though, because it involves the one player Kyle Shanahan’s offense has never really learned to live without.

Trent Williams hurt his shoulder during Thursday’s practice and is officially questionable for Sunday’s trip to AT&T Stadium.

For a team that has kept stacking wins with key pieces sidelined, this has quickly become the injury that matters most.

Shanahan said a shoulder injury late in the week is never something a coach feels good about, and he noted that “Sunday comes fast.”

He added that he hopes the few days between Thursday’s practice and kickoff will be enough for his longtime left tackle to be ready.

Asked how much Williams means to the offense, Shanahan kept it simple: “The left tackle, in general, is always huge.”

Anyone who has watched San Francisco’s run game over the past several years knows that it undersells how much Williams, in particular, has driven what this offense does.

If Williams can’t play, Vederian Lowe would take over on Brock Purdy’s blind side. Lowe isn’t a stranger to starting, since he made 25 starts over the previous three seasons in New England.

Still, there’s a huge gap between a capable fill-in and the player who has anchored the most efficient running scheme in football.

The outside-zone game that sets up play action, the screens that put playmakers in space, and Purdy's ability to trust his left side without a second thought all depend on Williams holding his ground.

A downgrade at that spot changes how Shanahan calls the game and how much attention his other linemen have to give the left edge.

Deebo Samuel is cleared to return from the hamstring injury that has kept him out since early October, and Keenan Allen is expected to make his San Francisco debut after the midweek trade with Indianapolis.

Mike Evans, meanwhile, will miss a third straight game with Achilles tendinitis. Working new and returning pieces into the passing game is hard enough with a full-strength line, and it gets much more complicated if Purdy is under pressure off the left side.

Williams isn’t the only veteran in doubt. Dre Greenlaw tweaked his knee in that same Thursday practice, though he was back on the field Friday.

Both starting cornerbacks, Renardo Green and Deommodore Lenoir, are also questionable after getting banged up late in last week’s win over the Raiders.

Dallas comes in at 4-5 and is looking for some momentum, and the injury report gives the Cowboys a little more hope than their record might suggest.

Even so, every snap Williams misses shows how much this team relies on him, and a franchise chasing the NFC’s top seed will be watching closely when the inactives are announced Sunday afternoon.