Koji Uehara
It may seem strange to include a closer who only spent four seasons with the Red Sox, but in those four seasons Koji Uehara was one of the best closers in all of baseball. His seasons in Boston, which spanned 2013-2016, were the best of his career and its no hyperbole to state that he was one of the main reasons the Red Sox won the World Series in 2013.
Koji’s 2013 season alone is worthy of inclusion here. During that magical year, he was 4-1 with a 1.09 ERA, 101 strikeouts, 9 walks, and 21 saves in 74.1 innings pitched. His WHIP was a miniscule 0.565 and at one point during the season he retired 37 straight batters.
He was even better in the postseason, going 1-1 with a 1.00 ERA, 16 strikeouts, and 7 saves in 13.2 innings of work across the ALDS, ALCS (of which he was the MVP), and World Series. His seven saves tied the major league record for a single postseason and he recorded the final out of the World Series by striking out Matt Carpenter of the St. Louis Cardinals to end Game Six.
What made all of that even more remarkable was that he was 38 in 2013 and was dominant at an age when most pitchers are already out of the league. For his four Red Sox seasons, Uehara went 14-13 with a 2.32 ERA, 291 strikeouts, and 79 saves in 226 innings.