Unfortunately, subbing in FIFA is kind of a tedious thing. You have to pause, and swap your guys, then get back in game. As stated above, the Kinect sub feature works great ... if you play with your volume on low or with headphones.
The good part of about having to go in to the tea management screen is you have full access to everyone stats, so if you sub on a corner, you can look at the stats and sub in a striker who maybe wouldnt be your first choice, but is tall with good heading or something to that effect.