Injuries are an inherent part of football.
The difference between winning and losing games regularly comes down to whether a team is fortunate enough to avoid the injury bug. The sting of losing a healthy player is especially disappointing when it occurs during the preseason, when hopes and anticipation for the upcoming campaign are high.
The Green Bay Packers learned Monday that Pro Bowl wide receiver Jordy Nelson will be lost for the season after sustaining a knee injury in Sunday's game against the Steelers, and the bad news doesn't stop there. Already this summer, Carolina Panthers wide receiver Kelvin Benjamin (ACL), Miami Dolphins safety Louis Delmas (ACL), Baltimore Ravens safety Matt Elam (biceps), New York Giants safety Bennett Jackson (ACL) and Minnesota Vikings offensive tackle Phil Loadholt (Achilles tendon) have been ruled out for the year.
Earlier this month, Philadelphia Eagles linebacker Travis Long had the misfortune of enduring his third ACL tear since 2012. Pittsburgh Steelers kicker Shaun Suisham sustained an ACL tear in this season's Hall of Fame Game. His replacement, veteran Garrett Hartley, had missed the entire 2011 season after sustaining a preseason hip injury as a member of the New Orleans Saints. Jacksonville Jaguars defensive end Dante Fowler Jr., the No. 3 overall pick in the 2015 draft, tore an ACL in his first workout at rookie minicamp in May.
With those injuries in mind, ESPN.com revisits some of the most notable NFL preseason injuries that forced players to miss an entire season:
Stone Johnson, Kansas City Chiefs, 1963: Undeniably the most tragic preseason injury, Johnson died less than two weeks after sustaining a severe neck injury while blocking on a kickoff return in an AFL exhibition game against the Houston Oilers. Johnson was a rookie running back from Grambling who had competed in the 1960 Olympics as a sprinter. The Chiefs retired uniform No. 33 in his honor.
Bubba Smith, Baltimore Colts, 1972: The former Michigan State star was entering his sixth pro season when he sustained a major knee injury after being entangled in the down and distance chain on the sideline during a preseason game against the Steelers in Tampa, Florida. He later lost a lawsuit claiming negligence against the NFL, two game officials and the Tampa Sports Authority. Smith went on to play four more NFL seasons with the Raiders and Oilers.
Darryl Stingley, New England Patriots, 1978: Stingley was entering his sixth NFL season when his neck was broken in a preseason game against the Oakland Raiders. Stingley was left a quadriplegic after a crushing blow to the head by Raiders safety Jack Tatum. No flag was thrown, and Patriots coach Chuck Fairbanks said he didn't think the hit was illegal.
Gary Danielson, Detroit Lions, 1979: The former Purdue quarterback was entering his fourth pro season when he was felled by a knee injury sustained in a preseason game against the Colts. Lacking a capable replacement, the Lions went 2-14 to earn the right to draft running back Billy Sims with the No. 1 overall pick in the 1980.
John Friesz, San Diego Chargers, 1992: The third-year quarterback tore knee ligaments in a preseason game against the Cardinals. The injury prompted general manager Bobby Beathard to acquire Stan Humphries from Washington as a replacement. The Chargers, under rookie head coach Bobby Ross, started the season 0-4 but won 11 of their last 12 games behind Humphries to become the first NFL team to lose its first four games and make the playoffs.
Ki-Jana Carter, Cincinnati Bengals, 1995: The No. 1 overall pick out of Penn State, Carter was felled in his rookie preseason by a torn ACL in a game against the Detroit Lions. He returned in 1996 but was never regained the dynamic ability he had shown in college.
Jason Sehorn, New York Giants, 1998: The cornerback tore ligaments when he was tackled by Jets defensive back Chris Hayes while returning the opening kickoff in a preseason game. Sehorn returned to play five more years, including the Giants' 2000 NFC Championship season.
Trent Green, St. Louis Rams, 1999: Signed by the Rams after two seasons with Washington, Green was a key piece to coach Dick Vermeil's plans. When he tore knee ligaments on a hit by Rodney Harrison in a preseason game against the Chargers, the Rams turned to a little-known grocery stocker named Kurt Warner. The rest, of course, is history, as Warner led the Rams to victory in the subsequent Super Bowl and launched the Greatest Show on Turf era in St. Louis.
Ryan Leaf, San Diego Chargers, 1999: Entering his second pro season, the former No. 2 overall pick tore the labrum in his throwing shoulder during the first workout of training camp. Leaf returned to action in 2000 but won just one of the 12 NFL starts he made after the injury. Leaf's post-NFL troubles have been well documented.
Jamal Lewis, Baltimore Ravens, 2001: As a rookie, Lewis helped the Ravens to a Super Bowl championship in the 2000 season. He tore his left ACL on a hit by teammate Kelly Gregg in a preseason practice months later. Lewis would play eight more pro seasons, and he rushed for 2,066 yards for the Ravens in 2003.
LeCharles Bentley, Cleveland Browns, 2006: Coming off his second Pro Bowl in four seasons with the Saints, the center ruptured a patellar tendon on his first play of training camp after signing with his hometown Browns. The injury marked the end of his playing career.
Osi Umenyiora, New York Giants, 2008: Fresh off a surprising Super Bowl win over the previously undefeated Patriots, the Giants lost their Pro Bowl pass-rusher to a torn meniscus in a preseason game against the Jets. He returned to the field in 2009 and won a second Super Bowl with the Giants in the 2011 season.
Jammal Brown, New Orleans Saints, 2009: The left tackle, a two-time Pro Bowl selection, sustained a hip injury in the preseason, and the Saints went on to win the Super Bowl with Jermon Bushrod stepping in as his replacement. After he recovered, Brown was traded to Washington in 2010.
Elvis Dumervil, Denver Broncos, 2010: Coming off a season in which he led the league with 17 sacks, the defensive end was lost for the season after incurring a torn chest muscle in a practice drill. He returned for the 2011 season and has earned Pro Bowl selections in three of the past four years.
Terrell Thomas, New York Giants, 2011 and 2012: The cornerback tore his right ACL in consecutive preseasons, missing the subsequent seasons both times. He had previously torn the same ACL in college at USC. He returned to play in every game for the Giants in 2013, but he retired earlier this year after going unsigned in 2014.
Bryan Bulaga, Green Bay Packers, 2013: Bulaga, a first-round pick in 2010, incurred a torn ACL in a preseason scrimmage. He returned to start 15 games in 2014, although the Packers abandoned plans to convert him to left tackle from the right side.
Dustin Keller, Miami Dolphins, 2013: Heading into his first year with Miami, the tight end tore three knee ligaments on a hit by Houston Texans safety D.J. Swearinger. The Texans rookie later said he aimed low to avoid being fined for a blow to the head under the NFL's revised tackling rules. Keller hasn't played since.
Jeremy Maclin, Philadelphia Eagles, 2013: The veteran receiver out of Missouri tore an ACL in a noncontact practice drill. Upon returning for the 2014 season, Maclin enjoyed his best year as a pro, posting 85 receptions, 1,318 yards and 10 touchdowns. He signed with the Chiefs for this season.
Mark Sanchez, New York Jets, 2013: The polarizing quarterback, picked No. 5 overall in the 2009 draft, suffered a torn labrum on a hit by Marvin Austin in a preseason game against the Giants. Jets coach Rex Ryan faced criticism for the injury, as Sanchez was hurt in the fourth quarter while playing behind a second-string offensive line. Sanchez signed with the Eagles last year, starting eight games in place of the injured Nick Foles.
Kiko Alonso, Buffalo Bills, 2014: After playing every snap during an impressive rookie season in 2013, the linebacker tore an ACL during an offseason workout prior to training camp. He was traded to the Philadelphia Eagles this spring, where he's reunited with his college coach at Oregon, Chip Kelly.
Sam Bradford, St. Louis Rams, 2014: The injury-prone former No. 1 overall draft pick tore the ACL in his left knee on a hit by Armonty Bryant in a preseason game against the Browns. Bradford, who tore the same ACL in a regular-season game in 2013, was traded to the Eagles in March and is poised to start for Philadelphia this season.
Darnell Dockett, Arizona Cardinals, 2014: The three-time Pro Bowl defensive end was lost for the season when he tore an ACL in a preseason practice. Dockett signed with the San Francisco 49ers this spring.
Sean Lee, Dallas Cowboys, 2014: The linebacker tore an ACL and meniscus in a noncontact team workout in May prior to his fifth pro season. He's poised to return to Dallas' starting lineup this season.
Fortunately for some of the players on this list, medical advances have made it possible for many athletes to return to top form after serious injuries, noted by ESPN.com's All-ACL team, a compilation of stars who recovered from knee reconstruction surgery.