The San Francisco 49ers may be without Brock Purdy on Sunday.

49ers coach Kyle Shanahan announced Wednesday that Purdy had entered concussion protocol. He started showing concussion symptoms on the plane back to the Bay Area after the team's 22-17 loss to the Minnesota Vikings on Monday night.

Though Purdy has not been ruled out of Sunday’s game against the Cincinnati Bengals, it’s unlikely that he will be able to clear protocols in time. No player that has entered concussion protocol this season has been able to play that same week.

If Purdy is unable to go, Sam Darnold will start in his place. Darnold landed in San Francisco this past offseason after a two-year run with the Carolina Panthers. He threw for 1,143 yards and had seven touchdowns and three interceptions in six games last season. He’s thrown one pass in four games this fall for the 49ers.

Purdy struggled against the Vikings on Monday night in Minneapolis, though it’s unclear what specifically led to his concussion. Shanahan guessed it happened on a quarterback sneak, but that he didn't realize anything was wrong until the plane ride home.

Purdy went 21-of-30 for 272 yards with a touchdown and two interceptions in the loss, which was San Francisco’s second straight after starting the season on a five-game winning streak. Christian McCaffrey scored twice in the game, too. That extended his streak to 16 straight games with a touchdown, which is one shy of tying the NFL record.

The Bengals have won two straight heading into Sunday’s matchup at Levi’s Stadium in Santa Clara and are fresh off a 17-13 win over the Seattle Seahawks.