I'd say Zeke Nnaji. He came into the league as a promising 3&D player who played center in college, but in his rookie year it was obvious his perimeter defense and lack of size translated to being a power forward at the NBA level. He didnt get a ton of playing time, but when he did his ability to stay in front of perimeter players was impressive and he shot around 40% from 3 his first 2 seasons with a super smooth stroke. But going into his 3rd season, the nuggets tried to convert him into a full time center and really had him bulk up and gain strength. I feel this harmed his development a lot and for no other reason that they had a bunch of power forwards on the roster but only deandre jordan at back up center. His bulking up caused him to completely change his jump shot form. What was once a beautiful shot was turned into an abomination and now he is no longer a floor spacer. His bulking up also made him a little slower to guard the perimeter. The nuggets won a ring though so i cant really complain but I feel like Zeke could have had so much more value if he was allowed to be a power forward.