Jack Flaherty and the Greatest Second Half

Before or during Jack Flaherty’s start today, viewers are likely to hear about his 0.91 second half ERA. It is the third-lowest second-half ERA since 1920. The second-lowest second half ERA belongs to Greg Maddux, who accomplished the feat in the strike-shortened 1994 season and pitched barely more than 50 second-half innings. The first belongs to Jake Arrieta, whose 0.79 ERA in the second half in 2015 propelled him to the Cy Young award. Of course, ERA alone doesn’t tell the whole story. For one thing, as with Maddux, it doesn’t show how many innings are being thrown. For another, different eras produce vastly different run-scoring environments. Pitching with a juiced ball or juiced players can make life more difficult for pitchers, rendering a lower ERA even more impressive. To that end, we can put Flaherty’s second half in perspective.

The easiest way to do so here at FanGraphs is to use RA9-WAR, which takes runs allowed, innings, and the run environment into account. Flaherty’s second-half RA9-WAR was 6.4, way out in front of Jacob deGrom’s second-place 4.8 mark. If you cut Flaherty’s RA9-WAR in half, he would still rank ninth in baseball since the All-Star Break. We have second-half splits going back to 1974; here’s where Flaherty ranks among the couple-thousand qualified second-half pitchers:

In 1976, Vida Blue threw 170.1 second-half innings with an ERA of 1.69; all those innings were enough to take the top spot. In 1974, Ferguson Jenkins put up a 1.59 ERA in 147 inning to nudge himself ahead of Flaherty. In the last 40 years, no pitcher has as valuable as Jack Flaherty in the second half when viewed through the lens of run prevention. As we know, more than just the quality of a pitcher goes into run prevention. When Arrieta went on his run in 2015, he had a great defense behind him, and Flaherty’s Cardinals defense has been pretty good as well. If we looked at the greatest second halves by the WAR that we typically use here at FanGraphs, Flaherty’s 4.1 is just behind Justin Verlander’s 4.4 mark this season and ranks 35th over the last 46 seasons. Instead of reproducing that list, let’s look at the pitchers 25 years old and younger:

Of the players on that list Flaherty’s age or younger, Eckersley and Blyleven are in the Hall of Fame, Gooden couldn’t live up to the promise of his early-career greatness, Tanana had a solid career, and Prior’s injuries ended his career early. Gooden is the only pitcher Flaherty’s age or younger with a better second-half WAR since 1974.

If we wanted to mix the run-prevention of RA9-WAR with a FIP-based WAR, which considers the results most in the pitcher’s control, an average or geometric mean of the two numbers should suffice. Here’s how those numbers stack together:

The 14th-best second half since 1974 might not have the same ring as the third-best, but it is still pretty darn impressive. There’s a fairly credible argument to be made that Flaherty’s second half was better than the one Arrieta put together four years ago. And there’s a very good case to be made that no player as young as Flaherty has ever produced a second half as impressive as the Cardinals right-hander. The Braves might be favored in this afternoon’s game, but Flaherty is unlikely to make it easy for them.