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The interest in Garza is a little cooler than that for Dempster. Understandable, as Garza isn't putting up nearly the numbers that his teammate is. But Garza is still good for some quality starts and has pretty overwhelming stuff.
Maybe his potential, age and the smaller load of prospects it would take to get him make him so attractive to these teams:
Boston Red Sox
According to Jayson Stark on Twitter, the Red Sox are very interested in Garza, but not so much on Dempster. The Sox can use a lot of help in the pitching department these days, so any move makes sense, really. Garza would provide a significant upgrade to pretty much anyone right now not named Jon Lester.
Bob Nightengale of USA Today thinks the Tigers are in on Garza. It's been widely publicized that Detroit would like to add an arm to complement Justin Verlander and the rest of a rotation that has inconsistency written all over it. When, not if, the Tigers get back to the playoffs, it would be huge to have an arm like Garza to throw a few key games.
Nightengale also mentioned the Rangers would love to snag Garza. Well, yeah. They'd also like to get Cole Hamels, Cliff Lee, Ryan Dempster, Nolan Ryan, Walter Johnson and Babe Ruth.
Texas seems to be in on every free agent these days, but I understand. A true ace pitcher might redeem the last two years, come October.
Kansas City Royals
This seems more like a very weak rumor, but word is that the Royals have at least entertained the possibility of bringing a guy like Garza (via Danny Knobler of CBS' Baseball Insider) to a decimated rotation.
It would be a huge boost to an awful rotation while they wait for some of their own pitching prospects to grow up in the minors. This would likely be looking forward to 2013 contention.
New York Yankees, Baltimore Orioles & Los Angeles Dodgers
I don't have any sources to link for you here—this is my gut feeling. I don't think any of them will end up with Garza (though if one of them were to snag him, I'd guess it would be the Yankees), but I'm not willing to count these three out. All of the teams need pitching desperately and two of them are absurdly rich. Time will tell!