When the San Diego Padres acquired Carlos Quentin from the Chicago White Sox, they gave up a couple of pitching prospects in Simon Castro and Pedro Hernandez. We’ve already discussed the trade in general, so now let’s focus more on Castro.

Right hander Simon Castro was signed as an international free agent out of the Dominican Republic back in 2006. He was quite raw, obviously, and didn’t really burst onto the big time prospect scene until after his 2009 campaign at Single-A Fort Wayne.

Castro pitched 140 and a third innings that year, posting a 3.33 ERA, 10.1 K/9, 2.4 BB/9, and .6 HR/9. Those are some pretty solid numbers, especially for a 21-year-old who had yet to really put it all together previously. After the impressive season, Castro ranked as the 57th best prospect in baseball by Baseball America, the #2 Padres prospect by Kevin Goldstein, and the #2 Padres prospect by John Sickels.

Sickels called Castro an “excellent arm with improving command” and Goldstein said that “he will be an above-average major-league starter, with some scouts projecting him as high as a No. 2.”

in 2010, Castro jumped up to Double-A San Antonio and continued to perform. He again pitched 140 innings (10 and a third in Triple-A Portland), with a 3.28 ERA, 7.3 K/9, 2.6 BB/9, and .6 HR/9. The drop in strikeouts was a bit alarming, but overall the 6-5 right hander had a more-than-respectable year in his first taste of Double-A.

The prospectors didn’t waver. Baseball America rated him as the 58th best MLB prospect while Kevin Goldstein and John Sickels both kept him at #2 overall in the Padres system.

Last season Castro started out in Triple-A Tucson and lasted only six starts, posting an ERA over 10 and ugly peripherals. He was sent back down to Double-A San Antonio where he would regain some of his form. His ERA was 4.33, but as we mentioned in the Quentin trade analysis, his peripherals looked largely the same:

One could argue pretty easily that he was actually a little better the second time around in Double-A, as his strikeout rate stayed virtually the same and his walk rate dropped quite a bit. His strikeout-to-walk ratio jumped from 2.97 in 2010 to 4.56 in 2011.

Of course, there are some legitimate reasons to be concerned with Castro’s 2011 year. For one, he struggled mightily again in Triple-A, giving him a 9.50 ERA, 1.5 HR/9, and nearly one walk for every strikeout in 36 career innings. It’s a small sample, sure, but it is ugly performance. Further, he repeated Double-A and while his peripherals were a little better, it’s not as if he dominated.

After 2011, though, would you expect him to completely drop off the prospect radar? That’s kind of what happened. Baseball America didn’t list Castro in its Padres top 10, Kevin Goldstein rated him #20 in the system, and John Sickels didn’t even rank him in his top 27.

Now, prospect lists are certainly fluid, and Castro didn’t do much to build on his 2010 campaign. Further, the Padres continued to add minor league talent from the draft and trades, helping push Castro’s freefall – not to mention, other guys establishing themselves.

Still, if you’re trying to look at the whole picture, it’s hard to believe that Castro should have fallen this far based on his ‘11 performance. If he was a top three prospect prior to 2011, there’s no way he’s a top 20-30 prospect after. That’s only my opinion, of course, and the prospect experts certainly study these guys closer than I do and have a lot of scout/front office contacts.

The general point, though, is not about prospect lists. Instead, the concern is over what Castro could still become. We’ve talked a lot recently about the Padres depth in the minor leagues. Castro was kind of a guy who embodied that depth. Formerly a highly-regarded prospect, there he was sitting way down — all but forgotten — on our organization depth chart.

Castro is gone now and while we received Carlos Quentin for him (and Hernandez), at this point it’s not clear that the acquisition was worth the potential cost.