WRONG!
There is something wrong with the game. I'm not sure if it was the latest roster updates but something disabled defense assist. I noticed this myself last night. I've tried resetting my settings, rosters and to no avail, they stand there as soon as you hold the button. It has nothing to do with the type of play you are using. If you pick man, he will follow his man if you hold down the button. If you choose zone, he will drop into zone and cover anyone that come across until he's out of his zone.
This is happening on the xbox too.