A backline, no matter how well drilled is only ever as good as its weakest link. If a defender is totally outmatched against an attacking player with the footwork and pace to simply dribble past them, then the goal will be in constant jeopardy. Two skills come into play here - firstly and most directly the ability to predict what the attacking player will do with the ball and tackle them cleanly, and secondly the ability to jockey back, slowing that attackers progress and allowing help to arrive.
A 1vs 1 drill with give coaches a very clear idea of where their defense is - and what needs to be improved on.
A goalkeeper is positioned in goal, with a defender placed inside the "D" on the edge of the penalty area. A line of six attacking players stand in the center circle with a ball for each. At the coaches whistle the first attacking player advances at pace toward the defender. The defender should stand his ground rather than charging forward immediately, but once the attacking player is close to him he has complete freedom of movement and decision.
The ideal result will be that the attacking player is tackled by the defender, almost as good as this would be to shepherd the attackers away from goal, slowing his speed and blocking any shot on goal. Get defenders to use their body position to encourage attacking players onto their weaker foot - offering them a channel to the left side of the goal for example so they must strike back across goal on their left foot.
Keep a tally of how your defenders do - they get one point for every shot the keeper has to save, three points for every goal. The defender with the lowest points total wins that session.
Watch how defenders use their timing and judgment - if they lunge for the ball and miss it, was it their error, or did the attacker "show" them the ball, knowing full well he could shift it away at the last moment? A great man once said you can't defend on your backside, so if this is happening repeatedly to the same player encourage them simply to remain on their feet, practice patience and be ready to backpedal - to jockey the attacker back, slowly neutralizing his pace. Speed is the attacker's friend - patience the virtue of the good defender.