"Nickel back" is a defensive player position in American Football. This term is only used for the game of football and not for anything else. Cox and McCain are competing for defensive back jobs. Teams usually have 4 defensive backs assigned to defense passing plays but sometimes employ 5 in passing situations. This 5th defensive back is called the "nickel back" because the American 5-cent coin is commonly called a "nickel". If Cox does not win the competition for starting defensive back, he will only play when 5 defensive backs are put on the field.

Relegate is correct not relagate. Relegate means reduce in rank. In a workplace, managers who can't figure out how to delegate , may get relegated to a lesser rank. The connotation is that , Cox is sure that he wins the competition, or else he should abandon selling french fries and start to be a rock star!