After a disappointing first half, the Cubs are all but guaranteed the NL Central crown after a good second half—they're 45-24, exceeded only by the 51-19 Indians on the strength of their 22-game winning streak.
The difference has been their offense. In the first half, per Fangraphs WAR, the Cubs had the 15th-ranked offense (a measure that includes fielding), literally in the middle of the pack. In the second half they've had the best offense, with 15.6 WAR, 1.7 more than the second-place Twins. By purely offensive measures they've been great: the best on-base percentage, the second-best slugging percentage (one-hundreth of a point behind the Twins), the highest weighted on-base percentage (wOBA), the highest weighted runs created (wRC+). After some bad luck and underperformance from key contributors in the first half, they're back to being the offensive juggernaut people expected.
Their pitching wasn't great in the first half, either: 13th in WAR. In the second half, they're down to 15th. But note the splits. In the first half, their starters ranked 18th, with a mediocre 4.66 ERA. In the second half—again, after some luck turned their way—they've ranked 8th, with a strong 3.45 ERA.
But the Cubs' bullpen hasn't improved. A relative strength in the first half (9th overall with a 3.26 ERA), it's fallen to 23rd in the second half, with the highest walk rate of any bullpen in baseball.
As far as pitching goes, they don't stack up very well against their likely competition (World Series odds via Fangraphs)
|Team||World Series Odds||2nd-Half Bullpen WAR Rank||2nd-Half Starters WAR Rank|
After the Aroldis Chapman acquisition last year, the Cubs went into the 2016 playoffs with a solid bullpen, close in overall performance to the Indians' bullpen but not nearly as flexible, which the Indians exploited in pushing the Cubs to the brink despite a diminished starting rotation. This year the Indians' bullpen is still great, and their now-healthy starting rotation is historically good.
The good news is that, of the great AL pitching staffs, only one can make the World Series. But the NL is fearsome as well—the Nationals, who had one of the worst bullpens in baseball in the first half, acquired Sean Doolittle and Ryan Madson from the Athletics in June, and they've run ERAs under 2.00 in the second half. The Cubs' midseason bullpen acquisition, Tony Watson, has walked more than a batter per inning since joining the team (including the two batters he faced last night against the Cardinals).
From the perspective of someone without a rooting interest in any of the teams, the 2017 playoffs should be great. There will probably be three 100-win teams, plus a team that lost 100 games last year, and the defending champions are coming off an encouraging second half. But if you thought last year was a high-wire act for the Cubs' bullpen, 2017 will keep you on the edge of your seat.