Every team in the league plays pick and roll. It is perhaps the most basic of plays in the game and when all else breaks down, it is the default go-to play. It is simple yet infinitely adaptable. Whatever you do to defend it, there's a counter move. Flash the ball handler and the roller runs free. Stay home and the ball handler has an open look or an open lane. Switch and you have a mismatch.
So if you can actually defend the pick and roll better than most, that makes you incredibly valuable. Which is a perfect description of Kevin Garnett. I'm not sure what the stats are this year, but in years past he's been singled out at as the best pick and roll defender in the league.
He's good enough to adapt to his ever changing supporting cast and Doc's tweaks to the defensive schemes.
Garnett's pick-and-roll defense has been typically effective throughout the year, but this season his general strategy has differed from the past. On high pick-and-rolls he used to hedge, jumping out and slowing down the point guard's progress. (Throughout his Celtics tenure, Garnett has been one of the two or three best big-man pick-and-roll defenders deploying this strategy.) This season Garnett has sagged back to contain the ball-handler, a strategy popular with most big men throughout the league. What separates Garnett from most of them is he's still quick and smart enough not to drop too low, which would give the point guard a wide open jumper or space to penetrate. Instead, more often than not he forces the ball back to the rolling man then slides a few steps across the lane to recover.
For all the intricate gameplans and adjustments and technical jargon, the game often boils down to fundamental execution and effort. Few put as much effort and detailed execution into their defense as Kevin Garnett does on a daily basis. And that's why your defense is always going to be better with KG on the court.