More evidence of the brain damage that occurs in professional football players was presented at the 2016 meeting of the American Academy of Neurology.See here.
Abnormalities in diffusion tensor imaging (DTI) and standard MR were found in a number of former NFL players . DTI reflects abnormalities in water molecule movement in white matter tracts. Dr. Francis Conidi studied 40 retired players 17 of which had positive findings on DTI .The average time in the league was 7 years and the average number of reported concussion was 8. Most of the players with findings were lineman. Linemen are less likely to sustain the very hard impacts that cause concussions and broken necks but just watch line play in a game for a few minutes and you can easily see the number of sub concussive head impacts that routinely occur.
addendum: 12/27/16 On a recent Sunday night football ( or was it Monday or Thursday) I noticed the pre game comments by the announcer pointing out that 3 of one team's starting lineup would not play because they were on the concussion "protocol". Several years ago those players would likely be in the lineup. The point being concussions are very common and a short while ago no one cared.