If the question is seeking to analyze on the field action, I think as previously asserted, the case has to be really egregious. The closest I could come up with would be the then- Tennessee Titans player Albert Haynesworth stomping on opponent Andre Gurode's face. The incident was so awful and heinous that the Nashville District Attorney considered pressing assault charges regarding play on the field. For the most part, legal authorities have respected the autonomy of league offices monitoring and policing the conduct of their athletes. Outside the field might be a different story. Indeed, a football player can be charged with assault. There have been plenty of examples of this. The most recent I can come up with is something that transpired in Knoxville, Tennessee with members of the Tennessee Volunteer Football team. Involving several members of the team, Sophomore Darren Myles, Jr. was charged with assaulting a police officer after a late night altercation at a bar. The immediate reaction was to dismiss the player from the team, while lawyers for the University argued that the athletes did not start the altercation that led to their arrests. In general, I would say that an athlete has some advantages and disadvantages in cases of assault or something of such a caliber. On one hand, they are able to post bail and enjoy the immediate advantages of legal counsel on behalf of the organization of which they are a part. Even the most flagrant cases of assault do not result in an organization immediately cutting ties with the athlete. In the case of the Tennessee Volunteer athlete Darren Myles, he was dismissed from the team only because this was his second offense in the span of three months. The disadvantages would be the publicity generated and that the public is, frankly, becoming fatigued with hearing stories of their beloved athletes involving themselves with the law in a negative fashion. The legal system does not make exceptions in charging football athletes, or any athlete, with assault when the circumstances warrant and the evidence merits.
People that play sports and especially contact sports assume a certain risk of bodily injury. They agree and consent to these risk's by virtue of playing the game. A hard tackle during a football game or a blow to the jaw during a boxing match are well within the rules of the game.
However, if some conduct is beyond the rules of the game it can be considered assault. For example, a deliberate elbow to the face during a basketball game or the deliberate use of spikes by a baseball player that is running the bases or eye gouging during football falls outside the normal inherent activities of the game and is considered assault. Any conduct that in intentional, deliberate, and violent that causes unnecessary injuries is by definition assault. This being said, professional athletes seldom pursue these charges.