SANTA CLARA, Calif. -- Entering an important matchup with the Green Bay Packers on Sunday, the San Francisco 49ers will be without starting quarterback Brock Purdy and defensive end Nick Bosa.

The Niners ruled out Purdy (right shoulder soreness) and Bosa (left hip/oblique) Friday afternoon, just before the team was scheduled to fly to Wisconsin. Coach Kyle Shanahan announced that veteran Brandon Allen will start in place of Purdy, with Josh Dobbs available as the backup.

With Purdy and Bosa out and left tackle Trent Williams (ankle) again listed as questionable, the 5-5 Niners could be without three of their most important players in a game they desperately need to stay in the postseason hunt.

"We're missing two good players, definitely," Shanahan said. "But we've got a lot of good players out there. By no means do we not have a chance to win. We're going to fight our tails off and expect this to be a real good game."

While Bosa missing a game to recover has been a real possibility for the past two weeks, Purdy's absence grew into a reality over the past 24 hours. Purdy was a limited participant in Wednesday's practice, but Shanahan and the Niners said they believed his sore shoulder would continue to get better as the week went on.

Purdy was on the field for some light throwing before Thursday's practice, but that clearly didn't go well as he retreated to the weight room for further treatment. That apparent setback surprised the 49ers.

The writing was on the wall earlier Friday when Purdy was nowhere to be found during the portion of practice open to media.

"This is something that we didn't think would happen early in the week," Shanahan said. "We were fully preparing for Brock to go and got a little surprised about this yesterday."

According to Shanahan, Purdy had an MRI on the shoulder Monday and it did not show any structural damage that would offer cause for concern that he could miss extended time. But Shanahan also said he was unsure when Purdy will be back.

"I don't want to say there's long-term concern," Shanahan said. "We thought he just needed some rest, and we really weren't concerned about [him] not being good this week. But when he started up Thursday, it just surprised him, surprised us with how it felt. So we had to shut him down.

"I really don't know what to think of it. ... But the way it responded this week, it's really up in the air next week. We'll have to see on Monday."

Shanahan said Purdy could not point to a specific play where he hurt the shoulder in last week's loss to the Seattle Seahawks. But Shanahan noted that Purdy's shoulder felt tight during the game and led to him spending more time warming in the game than he usually would.

With Purdy out, Allen is poised to make his 10th regular-season start since entering the NFL as a sixth-round pick of the Jacksonville Jaguars in 2016. Three of those starts were for the Denver Broncos in 2019, with the other six for the Cincinnati Bengals in 2020 and 2021. He has been with the 49ers since 2023, running the scout team against the defense for much of that time.

Allen said he didn't know until Friday that he would be starting, based on the expectation that Purdy would feel better as the week went on.

"It's an opportunity," Allen said. "The circumstances are what they are, but I think our team -- all year long we have been dealing with injuries here and there, and it's been a big next-man-up mentality. So it's definitely an opportunity for me to go out and play well and put our guys in a good position to win the game."

Shanahan said that despite his injury, Purdy will travel with the team to Green Bay. Bosa, however, will not. Bosa has been dealing with injuries to both hips and obliques, though the left side has been more bothersome this week.

Last week, Bosa played against Seattle despite pain in his right side but injured the left side with about nine minutes left in the third quarter. He did not return to the game and when it was over said he felt like he was "compensating" for the injury on the right side when he hurt the left.

Bosa didn't sound optimistic Wednesday that he would be able to play against the Packers, saying, "It gets better every day. We'll see."

In addition to Bosa and Purdy, the Niners will be without cornerback Charvarius Ward for a third straight game. Ward returned to practice this week after being away from the team to grieve the death of his 1-year-old daughter. Ward rejoined the team last weekend, but Shanahan said neither side wanted to rush anything because the cornerback hadn't practiced since before the Oct. 27 game against the Dallas Cowboys.

"We were just happy to get him back in the building this week," Shanahan said. "We don't want to put any pressure on him by any means to get ready to play. He just wanted to get back on the field. ... Hopefully he'll be good to go next week."

As for Williams, he played last week against the Seahawks after not practicing all week. He could do it again this week after again sitting out all three days of practice, though this time the 49ers have a long flight to Green Bay added to the mix.

Shanahan said a decision on Williams won't be made until "right up to kickoff." Tight end George Kittle, who missed last week with a hamstring injury, does not have a game designation and is cleared to return against the Packers.