HR 2 - With Brad Stevens' track record, how does he make a trade that's this bad?

Everyone in the Celtics organization is getting blame from ownership new and old to the GM making the moves. Why did Brad Stevens choose basically a mystery box over a new boat in Jaylen Brown? Then, it seems like a drastic turn for Stevens with all of the good moves he's made since taking the front office position and the good trades he's made up to this point. And, a look at the rest of the Eastern Conference and where the C's actually stand in the first days of the NBA offseason.