Craig Kimbrel is putting the finishing touches on one of the best reliever seasons in history. He has struck out 113 of his 226 batters faced this season, producing a 50% strikeout rate that nobody with 30+ innings in a season has matched or exceeded.
In fact, nobody has ever thrown 30+ innings with a 45% strikeout rate either. Kimbrel isn’t merely en route to establishing a new record. He is about to blow right by the existing record, which was set in 2003 by Eric Gagne. Gagne struck out 44.7% of the opposition in his Cy Young campaign. Only two other relievers have even topped 44% throughout history: Aroldis Chapman‘s 44.4% rate this season, and Kenley Jansen’s 44.0% rate last year. While relievers face such a small sample of batters, and another strikeout or two could materially affect the strikeout rates in question, Kimbrel still has a commanding lead. He has simply been unhittable this season and may have established the new benchmark for evaluating relief pitching performance in this era.
Kimbrel has thrown 61.1 innings over 61 appearances this year — both of which are down relative to his oft-cited over usage last season — and has allowed a whopping 26 hits. He has a .245 BABIP against and a 92.6% strand rate. His latter rate leads the league by a full two percentage points. He has also only walked 14 batters this season, which represents the third straight year in which his walk rate dropped. His walk rate was 18.2% in 2010, 10.5% last year, and now 6.2%, which is low before even comparing it to a gaudy 50% strikeout rate. His 8.07 K/BB ratio ranks 2nd all-time among pitchers with at least 30 innings and a 40% strikeout rate: Sergio Romo’s 70 punchouts against five walks last season has him beat.
One of the biggest reasons Kimbrel wasn’t going to succeed like this was his control: he didn’t have much of it as he progressed through the minors. Whether it be his own work, the tutelage of Roger McDowell, or both, Kimbrel has become a strikeout juggernaut that rarely gives up hits, limits free passes, and strands almost everyone that reaches.
His 1.03 ERA is far and away the lowest among senior circuit relievers. Chapman is in 2nd place at 1.55. Kimbrel also has a ridiculous 0.84 FIP to his name, which is not only substantially lower than Chapman’s 2nd-place 1.51 mark, but happens to be the lowest of all-time. Kimbrel’s 0.84 FIP narrowly edges Gagne’s 0.85 FIP in 2003.
Let’s not stop at the raw rates either, as changes in the scoring environment have made it tougher to score runs over the last few years. Context is key in any analysis and production relative to the league is more important than quoting raw numbers. Kimbrel has a 26 ERA- and a 22 FIP-. Gagne had a 30 ERA- and a 20 FIP- that season. Gagne logged more innings in 2003 — 82.1 to Kimbrel’s current total of 61.1 — but the two were essentially equals from a rate perspective.
Kimbrel has also kept balls on the ground at a 48% clip, up from last year’s 44% rate, and way up from his 28% rate as a rookie. He isn’t an extreme groundballer, but his rate is solid, especially in the context of his other rates. It has been historically tough to put a ball in play against Kimbrel this season, and almost half of the batters who didn’t strikeout or walk batted the ball on the ground. Be it weak contact or hitting the ball directly at fielders, a .245 BABIP with ~50% grounders is a very fruitful combination.
It’s difficult to evaluate relievers with WAR, especially across different eras, given the overall change in reliever usage. Relievers pitched more even 10-12 years ago, let alone 20-25 years ago, and WAR is a counting metric. Gagne’s 4.5 WAR is over a full win greater than Kimbrel’s 3.4 WAR, but that shouldn’t, in any way, invalidate what he has done this season. When great pitchers throw more innings, they produce more overall value.
However, a closer look at the post-1995 reliever leaderboards shows just how much per-inning value Kimbrel has generated this season. There have been 10 relievers with 3.4+ WAR in a season since 1995 and, aside from Kimbrel’s current season, the other nine ranged from 74.2 to 107.2 innings. That Kimbrel has produced 3.4 WAR with 13 fewer innings than anyone else in that leaderboard is remarkable.
Strictly using WAR, Kimbrel’s season might not seem like one for the ages. But he is literally doing things that nobody else in baseball history has either done, or come close to doing. That is the mark of a truly special pitcher, and a truly special season.