Porcello, 26, was acquired in a trade with the Detroit Tigers on December 11, 2014. He went 15-13 with a 3.43 ERA and 129 strikeouts in 204.2 innings over 32 appearances last season, including 31 starts. Along with leading the American League with three shutouts, he also placed among AL leaders while setting career bests in wins (T-8th), ERA (17th), innings (13th), double plays induced (2nd, 30), and fewest walks allowed per nine innings (8th, 1.8).
Signed by Detroit as the 27th overall selection in the first round of the 2007 June Draft, Porcello has made at least 27 starts and pitched at least 162.2 innings in all six big league seasons since his 2009 debut. For his career, he is 76-63 with a 4.30 ERA (513 ER/1,073.1 IP) and 655 strikeouts in 184 major league outings, including 180 starts.
Tazawa, 28, went 4-3 with a 2.86 ERA (20 ER/63.0 IP), 64 strikeouts, and only 17 walks and 58 hits allowed in 2014. He led the team with 71 relief appearances, tying his career high for a second straight season, and also topped Boston's bullpen with 16 holds. The native of Yokohama, Japan ranks fourth among major league relievers with a 5.3 strikeout-to-walk ratio over the last three seasons from 2012-14 (181 SO/34 BB).
Signed by the Red Sox as an international free agent in December 2008, Tazawa has pitched in 188 career big league games, including four starts, all with Boston in 2009 and 2011-14. He is 12-11 with one save, a 3.27 ERA (74 ER/203.2 IP) and 198 strikeouts compared to 44 walks as a major leaguer, and owns a career 2.94 ERA (60 ER/183.2 IP) in relief.
The Red Sox have two arbitration-eligible players remaining: left-handed pitcher Wade Miley and outfielder/first baseman Daniel Nava.