Thames had spent six seasons in the Yanks' minor league system before he was dealt. After being drafted in the 30th round of the 1997 draft, he was placed in Rookie ball, slowly moved up through the levels of the farm and was promoted to AA in the middle of 1999. After a marginal 2000 campaign, Thames broke out with a massive 2001 in which he jacked 31 homers for the Norwich Navigators and hit .321/.410/.598. His excellent season earned him a promotion to AAA the following year but he regressed badly, mustering a line of only .207/.297/.378. Despite those dismal results, he still earned a call up to the Major League club in June and famously debuted by hitting a home run of Randy Johnson.
Thames was traded at the behest of George Steinbrenner who was dead set on acquiring a left handed bat for the team. The 37 year old Ruben Sierra apparently fit that bill, but had only a 90 OPS+ over his last 6 seasons in the majors. The salaries were pretty much comparable and the Rangers decided to take a shot on Thames. Neither end of the transaction worked out especially well but the Yankees did slightly better. Sierra hit .276/.323/.432 (100 OPS+) in 189 plate appearances while Thames was fairly dreadful (47 OPS+) in 84 PAs for the Rangers. Thames was DFA'd by Texas at the end of the season to make room on their 40 man roster.
The Tigers signed Thames the following offseason. He provided solid value for Detroit over the first two years of the deal but was below replacement level in both 2008 and 2009. Last year, he missed two months with a severely pulled muscle in his rib cage. The Tigers elected to non-tender him instead of going through arbitration on his salary of $2.275M.
The deal with the Yankees could be worth up to $900K if Thames makes the Major League team. Not coincidentally, Randy Winn's base salary was $1.1M (with $900K in incentives, based on PAs) so regardless how the competition for the 4th outfielder in Spring Training goes, the Yanks will be paying out a maximum of $2M for that spot, all told.
Thames has a career .360 wOBA against lefties as opposed to .324 against righties, so he could be useful as a platoon option in right field. But as we are slowly learning, platoon splits are not always as pronounced as they seem. His value is derived from his power; he's hit an average of 33 home runs per 500 ABs over the course of his career. Thames provides an interesting depth option for the Yanks and with the structure of the deal, it's essentially a no-risk move. I don't think the team has any use at all for Juan Rivera, but at this point I wouldn't be totally shocked if they traded for him anyway.