What's the most important position on the football field, ONLY on defense?

Answer 1

Middle linebacker. He's kind of the QB of the defense... he gets the defensive call from the sideline and makes sure the rest of the defense knows the play and are lined up in the right spots. Some of the better middle linebackers have authority from the coaching staff to call an audible and change the defensive play if they see the offense in a certain formation..... kind of how the QB can change the play if he sees the defense showing a blitz formation.

The Colts and Giants game at the beginning of the year had a good example of this. While Peyton Manning was at the line doing his usual pointing and audible calls, Antonio Pierce (MLB for the Giants) walked almost up to the Colts side of the field to hear what Peyton was yelling. Then Pierce was turning and pointing the other defensive players into position and yelling his own audibles at them to counter what he heard Peyton calling.

Answer 2

Middle linebacker, in a 4-3 defense, or nose guard in a 3-4.

Answer 3

Middle linebacker. He needs quickness, both mental and physical, to read the offense and to rush. He functions as the field boss of the defense.