Giambi is enjoying a resurgent offensive year as a starter, sporting a .324 batting average with a 1.249 OPS and 10 homeruns in just 19 games. His numbers have been poor as a reserve, but he still provides some value as a backup first baseman or designated hitter for an American League contender.
Since he is signed for a very reasonable contract, the acquiring team wouldn't eat much salary. The Rockies could potentially acquire a C-level prospect, such as a high upside reliever in the lower levels. This move would be similar to the New York Yankees acquiring Lance Berkman from the Houston Astros before last year's deadline -- an aging slugger with power potential in the last year of his contract. In return, the Astros received a fringe infield prospect and relief pitcher Mark Melancon It's not a huge improvement, but Giambi was such a low-risk investment for Colorado that it would make sense to pull the trigger.
With the Rockies falling farther out of the playoff race, expect to see names beyond Jason Giambi appear on the trade market.