The 26-year-old Fien -- expected to be at Spring Training with Boston as early as Tuesday -- made nine relief appearances for Detroit in his only Major League season in 2009, giving up 10 runs in 11 1/3 innings for a 7.94 ERA.
But those numbers weren't indicative of a very solid tenure in the Minor Leagues.
Fien spent the other part of last season pitching for Triple-A Toledo, putting up a 3.41 ERA, a career-high 14 saves and 10.2 strikeouts per nine innings in 42 relief outings. In four Minor League seasons, Fien compiled a 14-6 record with a 3.04 ERA, striking out 239 and walking 49 in 222 1/3 innings.
During the 2009 season, Fien struck out 66 Minor League batters and walked just 15, ranking him second in the International League in strikeouts per nine innings (10.24).
Detroit designated Fien -- a 20th-round Draft pick in 2006 -- for assignment on Tuesday in order to make room on the roster for Johnny Damon.
Hernandez spent all of last year at Triple-A Tacoma, going 10-9 with a 5.23 ERA in 26 starts. Originally a third-round Draft choice by the Mets in 2004, the 23-year-old has played for the Mets, Marlins and Mariners.
Alden Gonzalez is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.