He's didn't declare because he knew he was getting picked in the draft, he declared because he was ready to go play professionally overseas with the maybe 10% chance he catches on somewhere in the NBA.
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People also have to understand that he's likely going undrafted, and that's probably the intention and plan for a while.
These 2nd round-type of players that have a million workouts with teams that aren't projected as late 1st rounders or early 2nd rounders, will have their agent tell teams in the mid/late 2nd round to not draft them as they likely already have a UDFA contract offered to them to sign minutes after the draft is over.
This place will explode on Thursday night if/when he goes undrafted, only for him to sign a vastly superior UDFA contract later that night compared to the majority of 2nd round contracts.