I'm not a huge SBW fan and I have been a big supporter of Nonu and what he's done since 08 but the stats of the last two games paint an ugly picture for Nonu's defence. Attack is one thing, nonu provides great energy on attack and a big workrate. But the WC is likely going to be won on defence, the knock out games are going to be all about defence.
honestly how can a guy go 80min and only make 4 tackles and miss two when the guy next to him had to make a game high 15 tackles. Something is wrong.
...and then SBW comes on and makes 4 tackles in 12 minutes; that's a rate of 27 tackles in a full match.
Now I am a fan of SBW and the way he plays, but its simply not fair on Nonu to make the comparison you have (Nonu 4 tackles in 80 minutes, Dan Carter 15 tackles in 55 minutes). The number of tackles you make in a game can very much depend on two factors
1. Your opponent's game plan
2. Your teams defensive strategy
If the oppo have decided to run at the inside channel a lot then 10 and 7 are going to get high tackle counts, and in the game to which you are referring (v Australia), that is exactly what happened, with Carter and McCaw both topping the count with 15.
Playing at 2nd 5/8 with Copper miss-passing a fair bit, its possible for Nonu to end up rarely being in a positoon to make a tackle, and you find the centre ends up making quite a few; and that too is what happened (Conrad Smith 7, SBW 4, together 11 tackles at centre).
So the statistics don't always tell the full story, which is why game analysts never rely on just the numbers in their analyses; in short, the stats support a conclusion but are not the sole provider of one.
Reminds me of an old saying (and a true one).
"You should always use statistics in the same way that a drunk uses a lamp post.... for support, not illumination"