My guesses are pace and length. That shot clock makes you run so much more. The average wingspan in the NBA has got to be 4 inches longer than the ACC.
His rebounding did translate. 8.3 boards as, essentially, the lone big on Duke's team isn't great. And that was with a few statistical outliers which drug up his average.
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Last year, Bags and Carter both averaged as much or more rebounds, while playing with another big, and being backed up by more bigs.
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Really, Oak wasn't a great rebounder at Duke given our roster, playing style, and his size, length, athleticism.
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Bigs today have to defend, rebound, and score. In that order. They can live without the scoring. Heck, if a big will rebound, that should be good for 4-6 points by itself, with another 4-6 thrown in on broken plays, dish offs, and alley oops.
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Off the bench, if Oak could get 10ish ppg, and 9ish rbp, he'd have a career. He could do more, but at this point it is fair to wonder if he has any intention of playing D.