What do we need Garza for? To waste his prime on a sub-par team? This team is going nowhere fast as currently constructed. Might as well trade him if you can get young, major league ready players. If we could get Rizzo and a top pitching prospect as the centerpiece, along with a couple other spects, I say do it.
The Garza trade made no sense in the first place as the team did not plan on contending last year. Now Hoystein needs to correct Hendry's incompetence. Everyone wants to see the Cubs win but are not willing to do the dirty work to build a solid foundation. Keeping Garza and signing Fielder will not make us a contender, it'll just keep a pitcher with a 3.50 era at a sub .500 record and inflate Fielders OBP to .450 when he gets walked every other at bat because he has no protection in the lineup.
If they could get Fielder, Darvish, Cespedes, keep Garza, and trade for a big bat to offer Fielder protection, then try to compete now. But I don't see that happening.
Might as well trade Garza at peak value and get some pieces to build along with Castro, Jackson, Cash, and McNutt. Then go and get Cespedes/Soler. Hell, maybe we could even convince Texas to take Soriano to DH in a trade if we take on salary but get some more prospects back.