[4] In September 2008, Tazawa announced his intention to skip the NPB draft in order to pursue a career with a Major League team.

While Tazawa attracted the attention of American scouts, NPB announced that the major leagues of two nations had a gentlemen's agreement against signing Japanese amateurs, and general manager Brian Cashman, whose New York Yankees had a partnership agreement with Yomiuri Giants, said his team considered Tazawa hands off.

[7] On March 22, 2009, Red Sox optioned Tazawa to its AA affiliate, Portland Sea Dogs.

After achieving a record of 9–5 in 18 starts with a 2.57 ERA with Portland, Tazawa was promoted to the AAA Pawtucket Red Sox on July 27, 2009.

[8] On August 7, 2009, Tazawa was activated from Pawtucket after the Red Sox designated John Smoltz for assignment.

In his third start, against the New York Yankees, Tazawa pitched six scoreless innings, allowing eight hits, two walks and striking out two in the win.

Tazawa started 2012 in Triple-A Pawtucket before being called up to Boston on April 18 to replace the struggling Mark Melancon.

By the end of July he had settled into the primary right-handed setup role ahead of new closer Koji Uehara.

On May 1, 2015, New York Yankees Slugger Alex Rodriguez hit a 3–0 93 mph fastball off Tazawa into the seats of Fenway in the eighth inning for his 660th career home run to tie Willie Mays for fourth place on the all time home runs list.

On August 11, 2019, Tazawa signed a minor league contract with the Cincinnati Reds and was assigned to the Double-A Chattanooga Lookouts.

[5] On July 20, 2020, Tazawa signed with the Saitama Musashi Heat Bears of the Baseball Challenge League.

[5] On December 26, 2020, Tazawa signed with Taiwan's Wei Chuan Dragons of the Chinese Professional Baseball League for the 2021 season.

On September 7, 2022, Tazawa signed with ENEOS, a team in the semi-pro Japanese Industrial League where he once played.

After graduating from high school, Tazawa led ENEOS to the 2008 Intercity Baseball Championship, and won the MVP Hashido Award.

While playing in Boston, Tazawa had used his ties to ENEOS to help the Red Sox cash in on a sponsorship deal with the Japanese motor oil company.