Darrius Heyward-Bey took a facemask to the helmet from Ryan Mundy, and looked to go limp immediately as he flew several yards through the air before hitting the turf. According to Steve Corkan of the Contra Costa Times, Heyward-Bey was taken to a local hospital via ambulance for examination, and observation.
Medical staff were being very cautious, leaving him on the turf for the better part of ten minutes while they stabilized his neck, and just removed him from the field on a back board. It's too early to speculate on the severity of the injury, though given the cautiousness of the staff it doesn't look good. However, he just gave a thumbs up to the crowd in Oakland, and appears to be moving which is a wonderful sign. Leading up to this point he was having a solid game, catching a touchdown early to keep the game in Oakland's reach.