Coming in to 2007, Kevin Youkilis had established himself as a useful, if brittle player in the major leagues. He had just finished his first complete season in the majors, playing a mix of third and first, and had accumulated a career .274/.379/.423 line. In 2007, at the age of 28, Youkilis had a career year, maintaining his batting eye, and adding a bit more pop. Although he was a useful player, he was hardly a star, especially given he was playing a premium offensive position and had never been a highly ranked prospect, as he put up moderate numbers in the minor leagues despite being old for the level at every stop. At the time, Baseball Prospectus wrote "He doesn't hit enough to play first base, and is an asset there only because he plays the position like a converted third baseman should, but he's locked in at the position following the Lowell re-signing. That leaves the Red Sox with a package comparable to Mark Grace or Wally Joyner at their peaks."
If you knew for certain that one of these players would hit .308/.404/.560 over the next three years and 18 wins over replacement, making him a star, while the other would hit a serviceable .300/.361/.459 and accumulate 5.5 wins over replacement, you'd pick the 21 year old prospect over the 28 year old journeyman every time, right? Well, baseball is a funny game. Youkilis developed an incredible amount of power, particularly for a player his age, while Butler's power plateaued in 2009 as a 23 year old and has been declining for a season and a half since then. Clearly Youkilis put some kind of hex on Butler, stealing his future career. Either that or someone did a hell of a job with Youkilis remaking his swing and approach to generate that kind of power.
As an aside, it makes me wonder a bit about what the future worry about Eric Hosmer, who is currently excelling with the Royals as a 21 year old, hitting an eerily similar .288/.341/.442. We'll see if Youkilis will steal his vital essence as well, to prolong Youkilis' career.