I'm not all that concerned with Rashard's post game, it's certainly spotty, and I think we can give him a few different looks to shake him a little on the nights that its on.
With Wesley Matthews: Anybody is going to need help on D with even a decent PG let alone someone as good as Rose. But Matthews is a good starting point, and having Dwight Howard protect the hole is going to help a lot.
Rashard Lewis has a very effective low post game against SFs.
The reason not to be overly concerned about it ... is questioning how often he'll really be there with Boozer + Z.Randolph on the team. How many touches in the post will he really get? Sure he'll get 3-4 touches down there and he'll do well with them ... that's a pain in the rear end but it's not going to kill you.
Furthermore, Rashard Lewis doesn't have the mentality to pick apart a team (in terms of scoring) that gives him a matchup advantage. Sure, he might do it in one game but he's not going to maintain it for a series.
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Why Wes Matthews instead of Baron Davis? Defending Derrick Rose I mean.