Underneath the bright lights, a sharp snap of leather breaks out amongst the crisp night air. The gaze of every player on the field focused on the football in flight. Unfortunately, the probability of all the players on both teams finishing the game healthy are against them. According to the NFL's injury report for 2017, the injury rate is 6.2 players per NFL game over the 2014 to 2017 seasons. These injuries are classified as injuries that result in removal from participation and inability to return to play, or inability to take part in full football activities following the injury.
In addition to frequent injuries occurring during the game, they also happen at practice, with an average of 894 injuries per year happening on the practice field. The most common injuries in the NFL are to the knee, ankle, upper leg, shoulder and head.
General manager and coaches around the league are well aware of the injury risk every time their team steps on the field. Each NFL roster is allowed to expand to 90 slots during the off season, and then before the regular season, they have to trim down the roster to 53 players. This allows them to examine potential prospects and ultimately to strengthen their roster depth, because it's just a matter of time before an injury strikes. This is why it's so imperative to not just have depth on your bench, but the highest caliber players possible. When the inevitable happens, a strong team will still have some good options to plug into the starting lineup.