Kent Mercker may have had the high billing coming up through the ranks but he ended up just being an average major league pitcher, same for Tommy Greene.
Sometimes, it works out that way. Don't remember and don't feel like researching to know if there were injury issues. Mercker was an underachiever.
For all the grousing about this or that trade, at this forum or at Scout, Bobby Cox made an enormous blunder on the Dale Murphy trade. Tommy Greene was included in the deal, and that would come back to bite them in a very big way in 1993. Same thing, it was over 20 years ago, so it's hard to remember what shortened his career. When he was effective, he was a stopper. Zane Smith had a longer career, but nobody would claim that he had a better makeup than either of these other 2.
FWIW, I saw both Mercker & Greene pitch for Richmond. Both had very good stuff, coming up. It sounds crazy to discuss it this way now, but instincts might've put these two at the "higher ceiling" than the other pitching prospects of the time, which includes one new HOF inductee and one soon-to-be.