I've seen how hard he plays on offense. I've seen him gamble on D, as opposed to playing smart and hard.
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The narrative among Duke fans is that Duke switched to a zone mostly because of the frosh bigs, but especially Bagley. Which has a lot of merit as an opinion. But that wasn't the only reason.
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In the M2M, opposing teams figured out a great strategy vs Duke. Spread out the floor, and run a PnR by whoever was being guarded by Bags and Allen. The end result of that was a great look at the basket, every time.
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Allen was better this year on D. No doubt about it. But he was horrible on D for the previous 2 years, with not enough data to form an opinion of his D as a frosh.
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Hustling after he got beaten, badly, on D doesn't equate to "effort." He was frequently slow on rotations, and had trouble staying in front of anyone, despite testing very well athletically. Further, he would gamble and try to jump passing lanes. When that works, great, because it leads to a run out. When it doesn't work, it leads to one guy being badly out of position, and 4 on 5 hoops. And, as mentioned, he didn't exactly rush back after a gamble.
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Yes, he would hustle around some, or even often, on D. He had too. Usually him rushing towards someone, often futilely, happened immediately after he'd been beaten, lost his guy, or blown a defensive rotation. He hustled to save face, and give the appearance of effort. Real effort would have involved not getting obliterated in the first place. Real effort would have involved knowing, and meeting, his defensive obligations within the scheme by at least, oh, say, the end of his sophomore season.
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Luke was the worst defender to ever start multiple games for Coach K. Allen was the second worst defender to ever start multiple games for K. And whoever is number 3 is still a lot better on D than Allen was. He was bad. And Luke at least had the excuse of being relatively slow and un-athletic. Allen is, ostensibly, pretty athletic. His failures on D could ONLY have been because of effort.