The NFL preseason is a formal preparation period for each upcoming season. It features exhibition games, training camps, and team practices. These allow players to learn the playbook and work on team chemistry while warming up for the regular season. The NFL regular season is short, so every game is important, making it crucial to be prepared right out of the gate in Week 1.While the preseason is useful in preparing for the regular season, it also presents a further risk of injury. The NFL is violent, so playing more football sometimes means an increased number of injuries. Teams hope that none of their players get hurt during the preseason, but it seems to happen every year. Here are five serious preseason injuries that changed the course of a player's career.#5 - Trent GreenSt. Louis Rams quarterback Trent GreenTrent Green signed a contract with the St. Louis Rams prior to the start of the 1999 NFL season to be their starting quarterback. Disaster struck when he suffered a season-ending knee injury during a preseason game and most thought the Rams' season was essentially over.The then-unknown now-legendary Kurt Warner replaced Green, and the rest, as they say, is history. Warner would go on to win two NFL MVP awards and a Super Bowl ring on his way to the Hall of Fame. Green would sign with the Chiefs two years later, where he had a mediocre career, posting a 48-40 record.#4 - Teddy BridgewaterMinnesota Vikings QB Teddy BridgewaterTeddy Bridgewater put together a solid rookie year for the Minnesota Vikings during the 2014 NFL season. He then improved even further during his second season, being selected to the Pro Bowl while helping the Vikings make the playoffs.Sportskeeda Pro Football@SKProFootballIn 2017, QB Teddy Bridgewater came in for Case Keenum to complete his comeback from a horrific knee injury while playing for the #VikingsTonight, the duo faces off as the #Browns take on the #Broncos37844In 2017, QB Teddy Bridgewater came in for Case Keenum to complete his comeback from a horrific knee injury while playing for the #VikingsTonight, the duo faces off as the #Browns take on the #Broncos https://t.co/MWznsKW1I6Bridgewater appeared to be well on his way to becoming a great quarterback, but he suffered a devastating knee injury prior to the start of the 2016 season. He missed the entire year and has never really been the same since as his upward trajectory was halted. He's recorded just a 16-19 career record since then and has never reached another Pro Bowl.