To let the computer AI defend and win the ball back for you, just press the "select/back" button to control the goal keeper. Just watch the AI wins the ball for you. After winning the ball, the AI will then let you control your player after a while. You can also press "select/back" button to switch back to your players.
Hope EA issue a patch soon to disable the keeper control. http://www.squidoo.com/fifa-12-tips
Use the call teammate button while containing the opponent player. Once your containing player is close enough, press the switch player button while holding the call teammate button.
By doing so, the AI player will be switch to your containing player, thus making the auto tackle for you.