Has there ever been a 1st rounder who got stashed in d-league for two years and later became an above-average NBA player?
Probably not a fair question given that the D-League is used now far more than it was in the past.
I think a better question is if there are other first round wings who have entered the league at a very young age (i.e. 18ish), done nothing for their first season or two in the league, and then turned into quality NBA players.
As for that question, the first that comes to mind is C.J. Miles. He entered the league at age 18, didn't show signs of being an NBA player until his third season, and didn't average more than 11.5 minutes per game until his 4th season. He's been a solid rotation shooter since then.
I imagine there are other examples.
As for guys who do nothing in their first year or two and then turn into an All-Star? I don't know of any, but I imagine there's at least one example.
Dorell Wright is another -- came into the league at age 19 and didn't do much his first few years, then blossomed into a quality role player, though he seems to have peaked a couple years ago.
John Salmon and Bobby Simmons both did very little in the league their first few years before blowing up as starters for a few seasons.