The Lakers are a horrid defensive outfit. The best thing you can really say about them is that they don't foul and send opponents to the line, and at the behest of one very lucky Wesley Johnson, they're able to block shots (don't expect this to last long). And while they have a knack for defensive rebounding, particularly with their bigs, everything about this screams fluke, and even then, they're still being out defensive-rebounded.
Many of the problems are age-related and start with the Lakers unholy quadruplet of guards--Steve Nash, Steve Blake, Jodie Meeks and Jordan Farmar. At first glance, Nash's defensive numbers look OK because he's really hidden all the time, but he can't defend any jumper any more as he overcompensates by trying to protect the paint due to his lack of wheels. The trickle-down effect is with Steve Blake, who at 6'3" has to defend a lot of shooting guards several inches taller than him: and he's just getting scorched individually, not only ceding all sorts of mid-range jumpers and threes but also losing his man for efficient at-rim conversions. Jodie Meeks has been similarly awful in defending shooting guards this season, relinquishing jumpers and at-rim finishes galore. Farmar is another one who struggles to contain key shots (at-rim shots, threes) especially when he has to guard shooting guards. Off-ball rotations may be decent with all four of them, but their inability to guard their supposed positions has led to a lot of hemorrhaging.
Elegantly stated, the Lakers lack a pure stopper. We knew this coming in given each player's body of work, but when Chris Kaman, a noted awful defender, is your best defender, it's pretty clear why your defensive efficiency is in the mid-20s. Kaman looks surprisingly reflexive with blocks and defensive boards in the outset, and has been surprisingly decent with his rotations and man-defense as well. The unfortunate part is, because of the guards' inability to guard the rim, Kaman has the lay low and thus opposing centers scorch him on mid-range J's. He also fouls a lot. But that's really less of his fault than on others. And the unfortunate part is, it's...Kaman. Meaning, we have to believe it's a fluke.
Pau's defense has fell off a lot. He just lacks lift. He can't guard centers, and even his rotations are very late at times. His rate of shotblocks are getting far less. There are a few virtues--he's gobbling up defensive rebounds, he still is able to guard power forwards, and he never fouls, per usual, at this point his lack of fouling looks lazy when coupled to his inability to defend. Still, he's better than the Lakers' guards, comparatively, due to his natural length and height. It's unfortunate because Pau's being misused on both ends--on offense he's settling for mid-range jumpers and getting fewer shots at the rim, and he's guarding centers. Worst of both worlds for him.
D'Antoni has used Wesley Johnson as a "faux" defensive stopper of sorts playing him at small ball power forward, but the results are very mixed. While Johnson is blocking shots to the stratosphere and corralling defensive rebounds well, he's an absolute hack, can't guard opposing power forwards at all due to weight limitations and doesn't really rotate all that well. Compared to a lot of the Lakers inferior defensive players, though, Johnson looks like a defensive god, even if what he's doing is far more sizzle than steak, a continuing trend ever since his rookie year. Shawne Williams is another who looks the part, with some ability to guard power forwards and rotate well defensively, but he cedes a ton of mid-range jumpers and threes to his man, doesn't rebound all that well, and is foul prone. And has a history of poor defense.
There's not much here. The Lakers don't force turnovers because they are one of the worst teams in stealing, but the problem is that it means that opponents can get any shot it wants against them, and if it's a jumper, it's over--the Lakers are 29th and 30th in mid-range and above break three defense. Even at-rim shots (20th) aren't a good proposition. There's just no personnel, and D'Antoni is making it worse by playing Steve Blake out of position and causing trickle down effects to the any of the bigs who did have some defensive potential.