If it's one from my lifetime then it's Ian Krol, Steve Lombardozzi and Robbie Ray for Doug Fister. Lombardozzi was gone before the season even started, Ian Krol was trash, and Robbie Ray had a terrible 2014 then the impatient Illitch demanded DD move him for a more ready player so Dombrowski dutifully moved him for Shane Greene who was a dumpster fire while Ray in his age-23 season put up a 3.52 ERA for the DBacks.
If it can be one from before my lifetime then it's the 1987 trade of 20-year-old John Smoltz for 36-year-old starting pitcher Doyle Alexander who was good for one year then fell off and disappeared.