Lakers won't be a defensive force. But they don't need to be. They just need to average 110+ points and stop their rivals on 105 points.
Lakers have many problems, but Nash will improve their defense, by simply improving their offense. Artest is the perfect example of a player that plays much better deffense when he has decent offense. I think that Gasol looks lost right now in the Lakers and if he will find his spot on the new Lakers on offense, it will translate on defense as well.
Not to mention that baskets on one side of the floor cut down the number of fastbreaks on the other end of the floor.
Let's not forget that Jamison, Lakers wild card from the bench, is now a starter. With Blake injured, suddenly the bench is Ebanks, Morris/ Duhon, Hill and Meeks. That's a bench that even Portland wouldn't trade their bench for...
If or when Lakers will be healthy, Lakers will have Blake, Jamison, Meeks and Hill - four decent bench players on the bench.
Nash should make Gasol and Dwight a lot happier, set Artest with some open looks and most important, he will take the pressure of Kobe.
I believe that if Kobe will know that he doesn't have to carry the team on his shoulder, he will gladly pass the torch (partially) and will play much better defense.
Also, Nash not the best defender, but he at least makes the opposition play some defense. Irving can't give Nash the space he gives Duhon/ Morris and if Irving is tired, he will score less and be less effective, simple as that.
I can count on one hand the number of times I took Lakers with the spread this season. I mostly look for good chances to play Over on Lakers or overall (and it's almost every day).