Preaching the National Football League’s commitment to a “culture of change,’’ one with greater concern for player safety, commissioner Roger Goodell noted in a speech at the Harvard School of Public Health Thursday that such change “in no uncertain terms . . . is our biggest challenge.’’

Addressing an audience of some 200 with a speech titled, “Leadership on the Road to a Safer Game,’’ Goodell talked of “changing the culture in a way that reduces the injury risk to the maximum possible extent — especially the risk of head injury.