The thing I've been dreaming would happen ever since it became clear that the A's season was lost has finally happened: Matt Holliday has been traded and the A's are receiving a premium hitting prospect in return. As has been rumored for days now, Holliday has been sent to St. Louis, with the main piece coming back being Brett Wallace. (Oakland also got an outfielder, Shane Peterson, and a pitcher, Clayton Mortensen in the deal; the A's sent along $1.5 million in cash to the Cardinals.)

Wallace showed in 2008 why the Cardinals made him their first-round pick, hitting a combined .337/.427/.530 in the Midwest and Texas Leagues. His power has dropped off this year, as he's hit .289/.368/.429 split between AA and AAA. Even with that, I'm sure he'll be up with the A's in September and has to be the odds-on favorite to be the starting third baseman on Opening Day, 2010. Wallace's bat is not doubted, but there is a question about his defense: will he be able to remain at third base? If he plays his way off of third, especially with Sean Doolittle and Chris Carter potentially on their way, the deal won't look nearly as good as it does now. But the A's obviously know their minor-league system and they've done their homework on Wallace. Maybe he won't be Eric Chavez or Jack Hannahan over there, but I'll place my faith in the Oakland scouts. (Kevin Goldstein, on the other hand, says that Wallace is a first baseman.)

Shane Peterson is a 21-year-old playing at AA, and the Cardinals' second-round pick in 2008. He's hit for a high average so far in the minors (.294 overall) with a solid walk rate (just shy of 10%), but little power (.120 ISO). He's played more in the corners, but he has logged 16 games in center, so he's probably not a lumber-y type (he's listed at 6'0", 195). His minor league numbers look not unlike those of Ryan Sweeney (albeit with enough walks to actually make him a potentially valuable OBP source, while Sweeney was a batting-average driven guy all the way).

Clayton Mortensen, finally, was a supplemental pick in the 2007 draft. He's already 24 and has put up mediocre ERAs and strikeout rates in the high minors, but his walk rates are ok, and Goldstein says he's a ground-ball machine. He is, in short, a throw-in that may amount to a fourth or fifth starter, a middle reliever, or nothing at all.