With the 26th pick in the first round of the 2005 Major League Baseball Draft, the Boston Red Sox selected Hansen and signed him to a four-year deal valued at $4 million.[7] The Red Sox added him to the 40-man roster, assigning Hansen to a conditioning stint in Fort Myers before sending him to the Double-A Portland Sea Dogs. Hansen joined the Boston Red Sox as a late season call-up, making his major league debut on September 19 against the Tampa Bay Devil Rays. Hansen struck out the first man he faced, pitching a perfect inning with two strikeouts, and would make three more appearances before the end of the season.[8]
After starting the 2006 season in Portland and Pawtucket, Hansen was recalled on June 6 by the Red Sox. On June 26, Hansen earned his first major league win in an extra-inning game against the Philadelphia Phillies. On August 20, Hansen entered a tie game against the New York Yankees in the 10th inning only to allow a game-winning home run to Jason Giambi and a two-run home run to Jorge Posada.[9] Hansen would end the season with a 6.63 ERA. During spring training for the 2007 season, manager Terry Francona suggested that the team may have rushed Hansen to the majors too quickly.[10]
In 2007 at Triple-A Pawtucket Red Sox, he went 3–1 with a 3.86 ERA but did not get a promotion to the eventual World Series Champion Red Sox. Hansen also played in the developmental Arizona Fall League to get extra work in after missing some of the season with forearm tenderness.[11] During the off-season, Hansen underwent surgery in the hopes of correcting the sleep apnea condition that may have affected his stamina and pitching performance.[12]
Hansen reported to Red Sox spring training in 2008 and initial reports suggested that the team was impressed with his performance.[13]
On August 27, 2008, the Pirates optioned Hansen to AAA (Indianapolis Indians). This move was made after Hansen allowed four earned runs without recording an out against the Chicago Cubs the previous night.[15] Hansen was recalled to the Pirates on September 2.[16]
On August 8, 2008, Hansen notched his 3rd career save, but first save as a Pirate, holding down a 2-0 Pirates victory over the Phillies.[17] Amazingly, all 3 of Hansen's career saves came in extra innings.
In 2009, Hansen was diagnosed with brachial plexus neuropathy, a rare condition causing pain, weakness, and numbness in the arms, shoulders, and upper back.[18] Recovery is usually complete, but can take as long as five years.[18] Hansen was able to pitch in a limited number of minor league games in 2010, but was not at full effectiveness, and the Pirates released him in early 2011.[19]
New York Mets
Hansen signed a minor league contract with the New York Mets on July 24, 2012.[20] The Mets released Hansen in March 2013.[21]