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Garnett's defense can be very good, but his offensive rebounding in a weakness.
Rebounding is an issue that impacts entire teams, the Celtics were the worst rebounding team in the NBA during the regular season last year.
In the playoffs they ranked 13th out of 16 teams, but Boston was actually sixth in defensive rebounding, so we all know what that means.
They stunk it up on the offensive boards.
No question about it, the Celtics averaged a dead-last eight offensive rebounds per game last season in the playoffs. That's bad, until you realize it was actually an incremental improvement over the 7.7 per game the Celtics averaged during the regular season.
There's a reason for those low numbers. The Celtics don't have any real low-post offensive threats. In order to get offensive rebounds consistently, a player really needs to be under the basket, and that's not where Kevin Garnett operates.
Garnett has become a deadly mid-range jump shooter. That has plenty of advantages, but it kills Boston on the offensive glass. When your tallest player, who also happens to be fairly athletic, is 18 feet from the hoop setting up for a jump-shot, an offensive rebound is clearly not priority.
That's probably why Rajon Rondo, who doesn't operate under the basket, but is so quick, routinely beats his defender and penetrates to the basket, was the Celtics leading per-game offensive rebounder in the playoffs.
Garnett isn't there, so he can't grab the rebound. That means that Garnett at center creates a serious issue for the Celtics when they're not hitting their shots, they won't get second chance points. Anyone who watched Boston labor on offense last season knows this is a problem.
Moving Garnett to center permanently certainly is not a solution to this problem either.