Concussion causes Henderson to sit out

The Vikings defense will need to be sharp against a 49ers offense that almost never shoots itself in the foot. That's why linebacker Erin Henderson's absence from Thursday's practice because of a concussion suddenly takes on such great significance.

San Francisco averages an NFL-best 167 rushing yards per game. The 49ers also have one of the league's premier tight ends in Vernon Davis, who has eight catches for 116 yards and three touchdowns. Translation: The Vikings will need a big day from their linebacking corps, which now might not be at full strength.

Henderson took a blow to the head late in last week's loss at Indianapolis, left the game briefly but returned. He wasn't formally diagnosed with a concussion until Wednesday, according to Vikings coach Leslie Frazier.

"There were no indications after the game, Monday, Tuesday," Frazier said. "And then Wednesday, he came after practice and just didn't feel right. So that's when they decided to take him through the protocol for concussions."

It's too early to know whether Henderson will be cleared to play Sunday against San Francisco. If he can't, Marvin Mitchell would take his starting spot at weak-side linebacker while Jasper Brinkley would inherit Henderson's snaps in nickel looks.

Henderson's status might not be as worrisome as the fact that his concussion wasn't diagnosed until midweek.

"I don't recall them telling me at all during the game that he had concussion symptoms," Frazier said.

On Jacksonville's go-ahead 39-yard touchdown with 20 seconds left, cornerback Chris Cook erred in letting Cecil Shorts get behind him. In the first quarter last week, Antoine Winfield stumbled with the ball in the air, misjudged Andrew Luck's deep throw and was easily beaten for a 41-yard reception by Donnie Avery. And then there was Reggie Wayne's 30-yard touchdown grab with 7 seconds left before halftime in which Henderson didn't drop deep enough and safety Mistral Raymond was late converging.

"Any way [it goes], you don't want it to happen. But I think it's just understanding the situation that we're in during the ballgame," defensive coordinator Alan Williams said. "We've said that we want to be situational smart football players, and I think that's taking time -- more time than we would like."

Etc.• Special teams coordinator Mike Priefer still questions whether safety Andrew Sendejo roughed Indianapolis punter Pat McAfee in the third quarter on what turned out to be a costly 15-yard penalty. "I think it's still a gray area and it's a difficult play to call," Priefer said.