Here’s another case illustrating the difficulty of recognizing concussions in the National Football League. The New York Giants’ No. 3 receiver, Mario Manningham, seemed normal after Sunday’s game and was OK Monday morning, according to the New York Daily News. http://www.nydailynews.com/sports/football/giants/2010/09/30/2010-09-30_manningham_tested_after_concussion.html But on Wednesday, Manningham began…