 Running back Gale Sayers, who had anchored Chicago's offense during the mid- to late-1960s, had suffered two knee injuries and was forced to retire during training camp in 1972.Star linebacker Dick Butkus was also hobbled by knee injuries and had to retire nine games into the 1973 season.Quarterback Bobby Douglass ran for 968 yards in 1972, setting an NFL record for quarterbacks, but passed for only 1,246 yards.