SANTA CLARA — Brock Purdy “definitely” will start for the 49ers on Sunday, so long as he clears the NFL’s final step in its concussion protocol, coach Kyle Shanahan said Friday.

“He was full go today (at practice), so he’s just got to pass that,” said Shanahan, who expects an outcome on Saturday. “Nothing’s been different.”

As encouragingly as Purdy practiced in back-to-back days, he must get cleared by an independent neurologist and the team’s medical staff. He is officially questionable for Sunday’s game between the 49ers (5-2) and the Cincinnati Bengals (3-3) at 1:25 p.m. at Levi’s Stadium.

Purdy, because he remains in the protocol, is prohibited from speaking with reporters.

“He’s been great, as he normally is,” said Sam Darnold, who’d start if Purdy can’t. “He’s prepared well, and, out in practice, he’s been slinging it and looking really good.”

Purdy played throughout Monday night’s 22-17 loss at Minnesota, then experienced concussion-like symptoms on the flight home and, thus, entered the NFL’s concussion. He progressed enough to be on the field for Wednesday’s offense-only work, then skipped that day’s official practice. On Thursday, he led quarterbacks through practice but was listed as limited.

In 2012, Alex Smith practiced throughout the week after a concussion forced him from a 49ers’ home game with the Rams. Doctors subsequently did not clear him for the following Monday night game against the Chicago Bears, resulting in Colin Kaepernick’s first career start. “I tried to push it and went out and practiced and, you know, it just wasn’t right,” Smith said in November 2012 of his post-concussion symptoms.

If Purdy ends his streak of 15 consecutive starts (playoffs included), then the 49ers would turn to Darnold, who went 21-34 as a starter with the New York Jets (2018-20) and the Carolina Panthers (2021-22) before signing a one-year deal with the 49ers when free agency opened in March.

Darnold said he’s approached this week as usual: “I’ve just got to go in there whenever, whether it’s from the jump or the middle of the game.”

Two years ago, Darnold sustained a concussion playing for the Carolina Panthers, and it did not prevent him from making his next start. Darnold said the key to the concussion protocol was to “just be honest to the training staff and doctors.”

“Sam is a professional, understands our playbook, gets us in and out of the huddle, and is efficient with the ball,” tight end George Kittle said. “I’ve seen him make some crazy throws. I remember against the Jets (in 2020), he had these crazy rolling out plays, and he was dicing us up late. I believe in Sam. He’s going to able to step into this offense if he needs to. And I still feel we have enough weapons around that we’re going to win with whatever we have on the field.”

When it comes to Darnold’s capabilities, there is a groundswell of confidence from both inside the organization and outside observers that he could be a capable fill-in for Purdy, who’s otherwise entrenched as the starter.

Smith, speaking Thursday on SiriusXM NFL Radio, said “it would be in rare territory” if Purdy gains clearance on a short week of preparation, seeing how the 49ers played Monday. Smith is among those who feel the 49ers are a great spot for Darnold to reboot his career, whenever that opportunity arises.

“Sam’s been in about as dysfunctional situations as you can be in to start a career,” Smith said Thursday on SiriusXM NFL Radio. “Take this from somebody … I’ve been through some dysfunction as well. He’s never been in a system like this, never been with a play caller like Kyle Shanahan. If there’s a place for him to go out and succeed, it’s here.”

John Lynch, the 49ers’ general manager, said Friday on KNBR 680-AM that Darnold is “a great fit for what we do.”

“Listen, this guy can throw it. Still incredibly young, in the big picture,” Smith added of Darnold, a 26-year-old product of USC. “They’re just fine if he has to go in and play. That’s the reality of the NFL. The chance your starting quarterback makes it through 17 games is rare.”

Steve Young, while not totally “absolving” Darnold of struggles in New York and Carolina, is also curious to see how Darnold responds, if needed.

“if Sam plays, I think there’s a lot of confidence that, well, maybe one game is not enough to show that (success) or maybe things go haywire, but if he had 10 games, I think you would say, `Steve, you’re right.’  And he is getting more help and he is capable of more than I thought,” Young said on KNBR 680-AM. “What that ceiling is, I don’t know. But we talk about big, strong guys running around throwing it all over the field; he can be that guy.”

NFL Films analyst Greg Cosell said he’d be “fascinated” to watch Darnold’s contrasting style to Purdy, noting how Darnold has “always had a little bit of a problem at times in what he sees, playing with a higher level of timing and anticipation. And we know this offense demands that.”

PRACTICE NOTES

Left tackle Trent Williams (ankle) is questionable, even though he has not practiced since his injury Oct. 15 at Cleveland, so Jaylon Moore looks in line to start a second straight game. … Wide receiver Deebo Samuel remains out as the 49ers are giving his shoulder’s hairline fracture time to recover through next week’s bye. … Running back Christian McCaffrey (oblique) was a full participant in all three practices and is fully expected to play.

Source: www.mercurynews.com