Delcarmen was a second-round pick by Boston in the 2000 Major League Baseball draft; he was the first draftee from a Boston public high school in 34 years. Beginning the season at Double-APortland, he posted a 4–4 record with three saves and a 3.23 ERA in 31 games. After a promotion to Triple-APawtucket, he struck out 12 with a 3.00 ERA over nine innings. In 2005, Delcarmen was called up to the major leagues for 10 games. He made his debut with the Red Sox on July 26, 2005. During the regular season, he pitched a total of nine innings with a 3.00 ERA and nine strikeouts. After starting the 2006 season in Pawtucket, Delcarmen was recalled to the major leagues on April 22. On June 11, Delcarmen got his first major league win, pitching two innings of relief against the Texas Rangers. Delcarmen was off and on the Boston roster during the 2007 season. He was recalled from Pawtucket on June 17, when relief pitcherBrendan Donnelly was placed on the disabled list. On July 23, Delcarmen finished out a 6–2 Red Sox victory over Cleveland with innings of scoreless relief, recording his first major league save. He finished the 2007 season with 44 innings in the majors and a 2.05 ERA. In the 2007 playoffs, he had innings pitched, allowing four earned runs, and was a member of the 2007 World Series championship team. After an intensive offseason workout regimen, Delcarmen lost. Manager Terry Francona remarked that "he’s lean and he’s looking really good." During the 2008 season, Delcarmen appeared in 73 games for Boston, recording a 1–2 record with two saves, 3.27 ERA, and 72 strikeouts. During 2009, Delcarmen's record was 5–2 with 4.53 ERA and 44 strikeouts during 64 games played. He started the 2010 season with Boston, where he appeared in 48 games with a 3–2 record, 4.70 ERA and 32 strikeouts. Overall, Delcarmen spent parts of six seasons with the Red Sox, appearing in 289 games while compiling an 11–6 record with three saves and 243 strikeouts in innings pitched.
Colorado Rockies
On August 31, 2010, Delcarmen was traded to Colorado for minor league pitcher Chris Balcom-Miller. Delcarmen appeared in nine games for the Rockies during the 2010 season, pitching innings while giving up six earned runs and striking out six.
Other MLB organizations
;Seattle Mariners On February 10, 2011, Delcarmen signed a minor league contract with the Seattle Mariners. He was released on June 1. ;Texas Rangers Delcarmen signed a minor league contract with the Texas Rangers on June 2, 2011. He was assigned to the Triple-A Round Rock Express. Delcarmen was released on July 13, after making eight appearances for the Express, posting a 6.75 ERA. ;New York Yankees The New York Yankees signed Delcarmen to a minor league contract on January 30, 2012. He also received an invitation to spring training. He spent the 2012 season with the Triple-A Scranton/Wilkes-Barre Yankees. ;Baltimore Orioles The Baltimore Orioles signed Delcarmen to a minor league contract on January 30, 2013. During the 2013 season, he appeared in 48 games for the Triple-A Norfolk Tides, compiling a 3–3 record with 2.83 ERA. ;Washington Nationals On December 2, 2013, Delcarmen signed a minor league contract, with an invitation to spring training, with the Washington Nationals. During the 2014 and 2015 seasons, he pitched for the Triple-A Syracuse Chiefs. He was released in June 2015.
;Bridgeport Bluefish On April 4, 2017, Delcarmen signed with the Bridgeport Bluefish of the independent Atlantic League of Professional Baseball. ;New Britain Bees and Retirement On November 1, 2017, Delcarmen was selected by the New Britain Bees in the Bridgeport Bluefish dispersal draft. On March 20, 2018, he signed with the team for the 2018 season. Delcarmen officially retired from professional baseball after pitching in a game for the Bees on May 31, 2018. He announced that he would be pursuing an opportunity as a studio analyst for the Boston Red Sox.
Scouting report
Delcarmen's career was on the fast track with a mid 90s fastball, and a great curveball—until he injured himself in May 2003. Delcarmen had Tommy John surgery, but returned to the mound in May 2004. Post-surgery, Delcarmen's fastball may have actually gained velocity, as he topped out in the high 90s. Delcarmen has decent control, with a decent changeup and a very good curve which he uses as his out pitch.
