SANTA CLARA, Calif. -- Less than 48 hours after their loss to the Minnesota Vikings on Monday night, the San Francisco 49ers had a surprise and significant addition to their injury report: quarterback Brock Purdy.

Coach Kyle Shanahan announced Wednesday afternoon that Purdy is in the NFL's concussion protocol after playing all of Monday night's game, leaving backup Sam Darnold to step in at practice while Purdy recovers. Purdy made it through Monday's game without any apparent issues but began feeling symptoms as the team was traveling back to the Bay Area and was placed in the protocol Tuesday.

"He started getting symptoms on the plane," Shanahan said. "We finally found that out when we landed. We were all pretty asleep, but then he got all checked up yesterday and now he's in the protocol."

Shanahan and the Niners were unsure when in the game Purdy suffered the injury, but Shanahan said there were no obvious signs when communicating with Purdy or in his performance. Purdy didn't take many hits, but he did attempt a pair of quarterback sneaks midway through the fourth quarter, and Shanahan acknowledged those plays might have been when it happened.

Video replay of the first of those attempts, which came on a third-and-1 with 7:03 left in the game, shows Purdy attempting to run behind left guard Aaron Banks to pick up the first down as Minnesota linebacker Jordan Hicks runs into the pile and makes helmet-to-helmet contact with Purdy.

After that sneak attempt, Purdy went 2-of-5 (not including a spike to stop the clock) for 20 yards with two interceptions to finish the game. The concussion diagnosis puts Purdy's status for Sunday's game against the Cincinnati Bengals in question. Shanahan said that, by the rules of the concussion protocol, Purdy does have enough time to clear it to be able to go against the Bengals.

The concussion diagnosis puts Purdy's status for Sunday's game against the Cincinnati Bengals in question. Shanahan said that, by the rules of the concussion protocol, Purdy does have enough time to clear it to be able to go against the Bengals.

Defensive end Nick Bosa went through the process in the playoffs after the 2021 season, sustaining a concussion in a wild-card win against Dallas and then returning for the NFC divisional round against Green Bay. He said Wednesday the process can be difficult.

"It definitely can weigh on you emotionally," Bosa said. "If it's a severe enough thing it can kind of send you into a bit of a depression if you do have a head injury. But for me it wasn't too bad. It's a pretty intense, drawn-out deal, which is good. But I passed all my tests. You have to go to an independent lady to do a bunch of cognitive stuff, and hopefully [Purdy] gets it done."

It wouldn't be the first time this season the Niners had a player suffer a concussion one week and play the next. Guard Jon Feliciano did it in Weeks 4 and 5, returning to step in for an injured Aaron Banks at left guard against the Dallas Cowboys after dealing with a concussion against the Arizona Cardinals.

But there's a smaller window for Purdy to come back because the Niners played on "Monday Night Football" and play again Sunday. Because of the short week, the 49ers had a walk-through on Wednesday afternoon.

According to Shanahan, Purdy was already feeling better Wednesday than Tuesday and would be able to participate in parts of the walk-through as he goes through the protocol. Shanahan added there isn't a set day in which Purdy must clear the protocol, citing a Thursday night game last December in which Purdy played on short rest without practicing.

"Brock didn't take a practice rep versus Seattle on 'Thursday Night Football' until pregame warmups, and he had to shut those down a little bit, too," Shanahan said. "So, he didn't really get a real rep that week until Play 1 and he played pretty good. So, probably stick with that."

If Purdy is unavailable, the Niners will turn to Darnold, whom they signed as a free agent in March and who beat out Trey Lance for the backup job. Darnold has played only in mop-up duty this season, but Shanahan said he believes Darnold is ready if needed.

"I have as much confidence in Sam as I could have with someone that I haven't gotten into a real NFL game with," Shanahan said. "He was great in the offseason. He's been great here in these seven weeks so far and he's always ready to go."

One other injury to monitor this week is left tackle Trent Williams' sprained right ankle. Williams did not play against the Vikings, in part with an eye toward this week's game but also because of the artificial surface in Minnesota.

Shanahan said Williams could practice as soon as Thursday, though there's no definitive timeline on his return.

"He wouldn't have been out there close to 100%," Shanahan said. "Hopefully he has a better chance to get there this week."