Already without their starting quarterback, the 49ers are optimistic that star left tackle Trent Williams will be able to play Sunday at the Packers, a source told ESPN's Adam Schefter.

San Francisco, however, also needs to see how Williams' injured ankle feels in pregame warmups to make sure he will be able to go, the source told Schefter.

If Williams is deemed fit to play, it would be welcome news for veteran quarterback Brandon Allen, who will make his first start in more than two years in place of the injured Brock Purdy (shoulder).

Williams, who is officially listed as questionable, did not practice this week after playing through his injury in last Sunday's home loss to the Seattle Seahawks.

The 49ers believe Purdy should be able to return next Sunday against the Buffalo Bills despite his shoulder soreness, sources told Schefter.

The Niners, however, also thought Purdy would be able to start Sunday against the Packers despite his injury, and he couldn't.

For the time being, there does not seem to be a high level of concern for Purdy's injury, sources told Schefter.

The Niners (5-5), who are fighting to stay in the postseason hunt, also have ruled out star pass rusher Nick Bosa, who is dealing with injuries to both hips and obliques.