Entering the 2016-2017 offseason, the Boston Red Sox lost Koji Uehara and Junichi Tazawa, both important parts of the bullpen since the 2013 World Series. With Craig Kimbrel having the worst season of his career up to that point (get ready for 2019 though) and the newly acquired Carson Smith not pitching in his first season, there were lots of question marks headed into 2017.
In order to stabilize the back-end of the bullpen, former Red Sox president of baseball operations Dave Dombrowski executed a trade with the Brewers to acquire right-handed reliever Tyler Thornburg on December 6, 2016.