Through 25 games this season, the Kansas City Royals bullpen is one that could potentially put up the best season of all time. Of course, the use of the modern bullpen is one that makes a conversation about the best bullpens of all time very difficult.
For example, the 1972 Pittsburgh Pirates had the second lowest bullpen ERA in the expansion era at 2.30, but only threw 219.1 innings as a bullpen. Three pitchers in that bullpen threw 54.1 innings or more, and that was it.
Compare that to last year’s Royals team that had four pitchers throw 59 innings or more out of the bullpen and you can see that bullpen usage has changed.