A football player will be more likely to be called offsides if "go before the snap!" is in his thinking, even if "don't" is part of the message that the coach says, . . . sometimes, especially if "don't" is what the coach (or the player himself) says.
Give the player a picture of what he is to focus on doing. "Keeping him 'green' will make him more mean!" ("Mean" playing football, that is.)