You look at Juan Cruz, with his impressive stuff, and his dominant numbers in AAA, and you think he should be a star. You hate to give up on him. But with the exception of one good year in Atlanta, it hasn't happened. He's been hit pretty hard in the majors. He's 27 now, and there isn't room for him on the A's roster. His trade value probably isn't going to get any higher than it is right now.
So the A's get Brad Halsey, who has some advantages over Cruz:
He's two years younger than Cruz.
He has more options remaining than Cruz.
He's cheaper than Cruz.
The A's will control his rights longer than Cruz.
He might be a better pitcher than Cruz.
He's more left-handed than Cruz.
If the A's do have an opening on their 25-man roster, it's for a left-handed relief specialist. Halsey has been a starter, but he does has some pretty nifty career numbers against left-handed batters: a .245 batting average against, and a 1.14 WHIP. If they decide not to use him as a reliever, he provides pitching depth at Sacramento, and another option to replace Barry Zito when he departs to free agency next year.
PECOTA projects Halsey to be worth about four more wins over the next five years than Cruz. Here's a comparison of their projected Wins Above Replacement Player:
Halsey's not a great talent, but he is a useful role player. With this trade, the A's get a better, younger, cheaper player who can potentially fill more roster holes for the A's than the player they lose. Unless Cruz suddenly matures into the player people thought he could become but never did, what's not to like?
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Just as I type this, Frank Thomas hits a monster home run in his second Cactus League AB. What's not to like?