Which 2015 lottery picks can still become breakout stars?

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Aside from Kristaps Porzingis and Karl-Anthony Towns, the top of the 2015 draft has been difficult to project through two NBA seasons.

Myles Turner and Devin Booker have turned out to be draft day steals, but elite high school class of 2014 prospects D'Angelo Russell, Stanley Johnson and Justise Winslow have had dramatic peaks and valleys during the early stages of their careers. (Jahlil Okafor, Emmanuel Mudiay and Mario Hezonja have also disappointed as they enter make-or-break seasons. We'll assess their futures in Pt. II).

How big of a role has team situation and infrastructure played in the lack of productivity and development of the once-heralded prep stars? Why exactly has each prospect stumbled thus far, and how much optimism should there be moving forward?

First-round picks from 2015 become extension eligible next July, making it imperative for teams to determine why these prospects have underwhelmed relative to expectations, and whether or not they should invest in them long term.