Friday the 13th came early for the 49ers.

Coach Kyle Shanahan revealed that two of the Niners’ most important players were suddenly questionable for their showdown against the Cowboys this Sunday.

The 49ers could be without All-Pro left tackle Trent Williams and stalwart linebacker Dre Greenlaw after both players suffered injuries in practice this week.

Williams tweaked his shoulder Thursday, and Greenlaw hurt his knee in the same session. Williams sat out Friday’s practice, while Greenlaw was listed as a limited participant.

Shanahan kept his cards close to the vest about either player’s status for Sunday.

“It just means that he’s questionable, whatever that means,” Shanahan said. “Anytime you hurt your shoulder on a Thursday, it doesn’t make you totally at ease. Sunday comes fast. But hopefully that’s enough time for him to be all right.”

Brock Purdy has been sacked only eight times this season, but if Williams, 38, isn’t able to go, he could have a gaping hole on his blindside. Led by Quinnen Williams, the Cowboys have racked up 12 sacks over their past four games, getting to the quarterback at least twice in each.

Williams’ absence would be “huge,” Shanahan said.

“The left tackle in general is always huge, and then a guy of Trent’s caliber is huge,” Shanahan said. “He’s a captain for a reason and everyone knows Trent Williams is one of the best players in this league.”

Veteran backup Vederian Lowe would likely step in for Williams. Enrique Cruz, a second-year seventh-round pick, would then be potentially one snap away from seeing his first live action.

Had it happened earlier in the week, Shanahan said the 49ers would have considered activating Austen Pleasants. The fourth-year lineman has been on injured reserve since the start of the season but “could” have been an option if he had been able to practice, Shanahan said.

“Vederian’s been doing great,” Shanahan said. “We’ve had Cruz working, so we’ve got the numbers there to handle it.”

Williams, in his 16th season, has been playing at as high of a level as ever. He has also been durable, playing every competitive snap so far this season.

The 49ers, on the other hand, already have experience playing without Greenlaw. When he was suspended for their Monday night win over the Commanders, rookie Jaden Dugger filled in and played well.

He managed to make an impact last week against the Raiders despite getting on the field for only 12 snaps, picking off Kirk Cousins and taking it to the house on the first play of the second half.

Shanahan said that “hopefully” the 49ers would have Greenlaw on Sunday.

“But if we don’t,” he said, “next man up. And our next man’s proven himself so far.”