What is an essential qualification for a player to switch from a defender to an offensive player during play?

To switch from a defender to an offensive player during a play, having possession of the ball is a fundamental requirement. This is because the player must control the ball to officially make the transition to an offensive role on the field. In American football, only the player who has possession can actively participate in the offensive play, which includes running with the ball or passing it.

In this situation, simply touching the ball does not grant a player the rights associated with being an offensive player, as they must possess and control the ball. Gaining 10 yards is irrelevant to switching roles; it's a measurement of advancement rather than a condition for changing player status. Similarly, being outside the neutral zone is related to positioning and eligibility but does not determine a player's role as offensive or defensive. Therefore, possession is the critical factor that allows a defender to become an offensive player during a play.