49ers' Brock Purdy could clear concussion protocol Sat., play Sun.

SANTA CLARA, Calif. -- San Francisco 49ers quarterback Brock Purdy has not yet cleared concussion protocol, but there is still a realistic chance he will start Sunday against the Cincinnati Bengals.

Purdy went into concussion protocol early Tuesday after feeling symptoms on the team's plane ride home from Minnesota late Monday night. Since then, Purdy has passed four of the league's five steps in the concussion protocol.

That has allowed Purdy to participate in practice on a limited basis Thursday and then be full go in Friday's session. Officially, Purdy is listed as questionable for Sunday, but he will attempt to pass the final step of the protocol on Saturday. If he clears that, he will start, according to coach Kyle Shanahan.

"That's the last thing he's got to do," Shanahan said. "Just one step left ... He was full go today; he's just got to pass that tomorrow."

The final step of the NFL's concussion protocol requires Purdy to be cleared for full contact by the Niners' team physician and be examined by an independent neurological consultant. If the independent consultant agrees with the team physician that his concussion has resolved, he can play against the Bengals.

Purdy appeared to suffer the injury on a quarterback sneak midway through the fourth quarter of Monday's loss to theVikings, when he and Minnesota linebacker Jordan Hicks had helmet-to-helmet contact in the pile. Purdy finished the game, throwing two interceptions in the closing moments.

This week, Purdy has progressed through the protocol with a notable step forward each day. In Wednesday's practice, Purdy had to be held out of parts where the team was in helmets, though that session was a walkthrough. On Thursday, Purdy had moved further along in the protocol and was able to practice on a limited basis before being labeled a full participant in Friday's practice, which is also generally a lighter session.

Sam Darnold, who would start if Purdy is unable to clear the protocol's final hurdle, said he got "a couple reps" with the starters this week, but that Purdy seemed to be recovering well as he took most of the work.

"He's been great, as he normally is," Darnold said. "He's preparing really well and out there at practice he's been slinging it and looking really good."

In 2021, Darnold went through a similar situation to Purdy, sustaining a concussion in Week 8 and then returning to play in Week 9. Like Purdy, Darnold said he was able to practice and then was cleared to play a day before the game. Darnold added that he hasn't changed his approach much this week given Purdy's uncertain status.

"[I've] just got to be ready to go in there whenever," Darnold said. "Whether it's from the jump or middle of the game, whenever. So, just going to handle the situation as I normally would and just prepare like I normally would and be ready to go whenever my number is called."

Purdy's injury status isn't the only one the Niners will be monitoring in the next 24-36 hours. Left tackle Trent Williams did not practice for a third straight day Friday as he continues to deal with a right ankle sprain. If Williams can't play for the second straight week, Jaylon Moore would replace him.

Given his body of work and veteran status, Williams is one of the players the Niners trust to play without practicing, so that decision could be made closer to game time on Sunday. One thing the Niners will have to consider is that their bye is coming after Sunday's game. There are two ways to look at that when it comes to Williams' availability, Shanahan said.

"That could be the reason you don't [play him]," Shanahan said. "It also could be the reason that we do. It was a lot harder last week because we felt if he did go there was no chance at this week. We definitely don't want to hurt him two weeks from now, but he also has two weeks to recover as opposed to 5 days. So, we weigh all that into account."

One other key player the Niners are hoping to get back from injury sooner than later is receiver Deebo Samuel, who is dealing with a hairline fracture in his shoulder. Samuel is out this week but is making progress, according to Shanahan.

"He's doing good," Shanahan said. "We have got to wait for that to heal, but he's healthy enough everywhere else so he's conditioning, working out, doing well and hopefully that will be good after the bye week."