Craig Kimbrel
Like Koji Uehara before him, Craig Kimbrel only spent three seasons in Boston and yet he was integral to the team winning a World Series. From 2016 to 2018 he was one of the best closers in the league and seemed unhittable until he apparently lost some of his effectiveness toward the end of 2018 and into the postseason.
His numbers during his three Boston seasons look like this:
2016: 2-6 W-L, 3.40 ERA, 83 K, 31 SV, 53.0 IP (57 games)
2017: 5-0 W-L, 1.43 ERA, 126 K, 35 SV, 69.0 IP (67 games)
2018: 5-1 W-L, 2.74 ERA, 96 K, 42 SV, 62.1 IP (63 games)
Toward the end of the 2018 season, it seemed as though hitters had figured out Kimbrel and while he remained effective, every appearance of his in the postseason that year was a rollercoaster. He recorded six saves across the ALDS, ALCS, and World Series, but none of them were easy and in most of them he was bailed out by some stellar defensive play behind him.
His 11.57 ERA in the ALDS attests to that; he was better in the ALCS (4.50 ERA) and World Series (4.15 ERA), but still shaky. Entering free agency after the season, he made his (frankly absurd) salary demands known and the Red Sox made a token lowball offer that he turned down before hitting the open market.
Still, the Red Sox wouldn’t have won three straight division titles or the World Series without Kimbrel and he was an All Star all three of seasons in Boston. He also recorded his 300th career save (becoming the youngest pitcher ever to do so) in 2018. He was the next in a lineage of great Red Sox closers following Jonathan Papelbon and Koji Uehara and deserves inclusion on this list.