Cue up your bad puns about "changing his Sox", but the fact remains: Matt Thornton has been traded from the White Sox to the Red Sox. In return, Chicago gets Boston minor leaguer Brandon Jacobs.
The 36-year old Thornton was traded away by the White Sox about two years too late, as he's not the dominant force he was from 2008-2010 anymore. This year, Thornton has the lowest strikeout rate of his career, and his walk rate is his highest since 2007. Thornton's always been primarily a fastball pitcher, but he's mixed in more sliders in recent years as his velocity has dropped. This season, the 94.1 mph his fastball is averaging is its lowest since 2005 with the Mariners.
Thornton's splits are also heading in the wrong direction, as after years of being effective against both lefties and righties, he's having much less success against righthanders, allowing a .314/.414/.420 line this season compared to just .170/.232/.385 against lefties. He's been used more and more in a specialist role by Robin Ventura this season, pitching just 28 innings in 40 games.
The Red Sox are probably hoping that Thornton works out better for them than the last former White Sox reliever they picked up (Bobby Jenks), and if he's used properly, that could very well be the case. The AL East has some really good lefty hitters (James Loney, Robinson Cano, Colby Ramus, and oh yeah, CHRIS DAVIS), and if Thornton is able to get them out at a better clip than Craig Breslow (who has been murdered by lefthanders this year), he should be able to slide into the role as Boston's primary lefty specialist with Andrew Miller out for the season following a sprained out.
Oh yeah, the return. Jacobs is just 22, and he ranked 13th among Fangraphs top 15 Red Sox prospects coming into the year (and ranked the same on Baseball America's rankings). He's been described as a work in progress and an unfinished product with solid raw power and baserunning acumen. He's spent nearly all of this season with Salem of the high-A Carolina League, where he hit .244/.334/.440 with 81 games, homering 11 times, stealing 10 bases, and punching out 88 times in 335 plate appearances. I've heard him described as a lottery ticket prospect, and that seems like a fair description of his abilities.
All in all, the Red Sox added another cog to their machine, one that was necessary after Miller went down. Thornton isn't the same guy he was in the past, but I don't think Boston is going to want to use him like the White Sox have used him in recent years. As for the White Sox, this is yet another reason that you should sell high on relievers. From 2007-2011, Chicago paid Thornton a grand total of $8 million, picking up a total of 8.8 fWAR in value. They paid him $5.5 million last year, and have already roughly paid $3 million this year, and will be paying $750,000 of his remaining salary this season, and got 1.3 fWAR in value along with a prospect who wasn't in Boston's top ten. If they dealt him in say, 2009, the return would have been far greater, and they would have spent less money in the process. Considering that the White Sox won just one playoff game during Thornton's tenure with the team, it's not a stretch to say that they would have been better off dealing him three or four years ago.