Bringing on a defender is usually when you're pretty desperate and start inviting them on, while building a wall in front of the goal to repel the attacks. By bringing on Malouda (whoever the name, just a midfield player) we press higher up the pitch, giving them less time on the ball... if we brought on a defender, we would have to take away a central or a wide player, leaving us more vulnerable in those places and give away iniative in midfield. + if they DID score, we would be chasing for the winner with 5 defenders and less men upfront, against a team like Sunderland it's not worth to risk it and we would all be on AVB's neck "why the fuck were we chasing the game with 5 defenders and give away the iniative?".