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On Saturday, an independent doctor cleared San Francisco 49ers quarterback Brock Purdy to play in Sunday's game against the Cincinnati Bengals, giving the Niners their starter back after what could be the quickest stay for a player in the NFL's concussion protocol this season.

Purdy complained of symptoms on the plane ride back from San Francisco's loss to the Vikings late Monday night/early Tuesday morning. He presumably went into protocol Tuesday, so getting out of protocol Saturday would represent what is possibly the shortest stint in the protocol in the NFL this year.

The median return from a concussion in nine days, according to a league source, though somewhere between a quarter to a third of players who suffer a concussion return the following week. More than a dozen players have gotten out of protocol in one week this season, a source said.

Brock Purdy
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Purdy took a hit to the head during a fourth-quarter quarterback sneak when Vikings linebacker Jordan Hicks delivered a helmet-to-helmet blow on the tackle attempt.

Purdy was not evaluated for a concussion after the hit, and he wound up throwing two interceptions after the play in the 22-17 loss.

Not every blow to the head requires a concussion check, and that alone isn't enough to trigger a check. The player must either exhibit or report symptoms, or a doctor or spotter must initiate the protocol.

League sources said because Purdy didn't exhibit any symptoms in the immediate aftermath of the hit, no protocol was initiated.

"The NFL/NFLPA concussion protocol relies on independent doctors and trainers who are experts in diagnosing and treating concussions," the league said in a statement provided to CBS Sports. "The experts watch for signs and symptoms of a concussion in a player's behavior following head contact which would necessitate further evaluation. Players, teammates, coaches and officials also contribute to identifying players in need of an examination.

"Three to four concussion evaluations are performed for each diagnosed concussion. This conservative approach safeguards player health. Occasionally symptoms develop on a delayed basis, so care following a game and the subsequent days is a part of the protocol as well."

49ers head coach Kyle Shanahan voiced optimism mid-week that Purdy would be cleared in time to play.

By Thursday, Purdy was already in step four of the five-step return-to-play protocol. According to a league document, Phase 4 involves club-based non-contact training drills.

"The player continues cardiovascular, strength and balance training, team-based sports-specific exercise, and participates in non-contact football activities (e.g. throwing, catching, running, and other position-specific activities)," the protocol states. "Neurocognitive and balance testing should be completed no later than the end of Phase Four with the results interpreted as back to baseline."

He practiced fully Friday with clearance from the team doctor, and then the final approval Saturday from an independent neurological consultant.