Vikings linebacker Erin Henderson missed practice for a third consecutive day Friday, still dealing with a concussion that he suffered late in last Sunday's 23-20 loss in Indianapolis. Henderson banged his head on a tackle of Reggie Wayne with a little more than 4 minutes to play. He was helped off the field by head athletic trainer Eric Sugarman but later returned for Indianapolis' final drive. According to Vikings coach Leslie Frazier, Henderson wasn't diagnosed with a concussion until Wednesday when he came out to practice and admitted he wasn't feeling right. Henderson has now been officially ruled out for Sunday's game with the 49ers. If he doesn't play, Jasper Brinkley would see more extensive playing time in the Vikings' nickel defense. Marvin Mitchell, meanwhile, would start in place of Henderson at weakside linebacker in the base defense. Mitchell missed the first two regular season games with a sprained ankle and now vaults right into a leading role. "He's done a good job in camp against the run and played the pass well," Frazier said. "And he has a good grasp of what we're trying to do from a defensive standpoint. What he'll miss is the speed of the game with this being his first week back. That will be an adjustment for him and hopefully won't take him a lot of plays to adjust to that part of it." The Vikings will have their hands full defensive Sunday trying to contain a San Francisco offense that has averaged 5.7 yards per rushing attempt in impressive early-season wins over Green Bay and Detroit. Frank Gore is the catalyst of the ground game and has 201 yards and two touchdowns on 33 carries so far this season.In addition, 49ers tight end Vernon Davis (eight catches, 116 yards, three TDs) continues to be a match-up nightmare. "Man, he is a talent," Frazier said. "I've watched him run by cornerbacks, run by safeties, linebackers. He's a very good tight end. You definitely have to pay attention to him." Besides Henderson, the only other Viking to miss Friday's practice was defensive end D'Aundre Reed, who's out with a calf injury and will also miss Sunday's game. Center John Sullivan was limited in practice Wednesday and Thursday but returned as a full participant Friday. He is probable for Sunday. Other Vikings listed as probable on the injury report are: Mitchell (ankle); Chris Cook (biceps); Brian Robison (elbow); Jarius Wright (ankle); Andrew Sendejo (ankle); and Rhett Ellison (ankle).