Alex Raskin and Devin Kharpertian take a look at Kevin Garnett's resurgence in 2014 and find it stunning, and not just on offense where it seems all of his mid-range shots are dropping. At it's really not on the boards either, where on a per-minute basis, Garnett is rebounding more this season than in any of his years in Boston. The big difference and it is BIG, is on defense where a lineup with KG is stunningly better than one without him.

As Raskin notes...

Entering Tuesday, the Nets had yielded 6.1 fewer points per 100 defensive possessions when Garnett had been on the floor. Surprisingly enough, the Celtics were just 4.2 points better per 100 defensive possessions while Garnett was marching toward his 2008 defensive player of the year award.

Kharpertian puts it another way...

n sum: with Kevin Garnett on the floor, the Nets make the league's best defense look average. With Garnett off the floor, the Nets look like the league's worst defense. It's that significant.
Of course, the same is somewhat small. Garnett is averaging only 22 minutes a game and has missed five, mostly for rest, but you don't need a lot of numbers to tell the team is defending better with Garnett anchoring and communicating.