Gale Sayers
The Chicago Bears running back, whose friendship with teammate Brian Piccolo was immortalized in the 1971 tear-jerker “Brian’s Song,” also saw his own playing career end before its time. Sayers was Rookie of the Year in the 1965 season and led the NFL in rushing yards in 1966 and 1968, but injuries to both knees meant that he only played four complete seasons, and he ultimately retired at age 29 during the 1972 preseason.