This is taken off Wikipedia.....

The move to leave Schiraldi in the game, however, was not as widely criticized as the one to leave Buckner in. Buckner had been suffering from ankle problems since requiring two surgeries (one for each ankle) to fix injuries over a decade earlier. The Red Sox acquired Buckner during the 1984 season and used Dave Stapleton, who had been the team's first baseman prior to the acquisition, as a late inning replacement for Buckner. When initially asked why Stapleton stayed on the bench, McNamara said that he felt that Buckner deserved to be on the field if the Red Sox were to hold on and win the game.[20] Stapleton was not happy about remaining on the bench and in 1995, he blasted McNamara in a book written about the '86 Red Sox. McNamara responded in the same 2011 interview with Costas that the real reason why he did not put Stapleton in the game was because Stapleton was known by his teammates as "Shaky", in reference to his lack of stout defensive play.