Why the Cubs won't win the World Series

A seven-walk outing on Aug. 18 was the start of a downward slide for Jake Arrieta. AP Photo/Charles Rex Arbogast

The Chicago Cubs finished the season with the best record in the major leagues (103-58), third-best offense (5.0 runs per game), best pitching staff (3.15 ERA), best defense (92 defensive runs saved) and best run differential (plus-252).

So what could possibly derail this team and stop the Cubs from winning it all?

It’s simply not probable

According to BaseRuns, a metric Fangraphs uses to estimate team performance, the Cubs were five games better than any other team in baseball. They also finished 8½ games ahead of their closest pursuer in the National League Central.

Even with that, Fangraphs gives the Cubs a 17.6 percent chance to win it the World Series, second to the Los Angeles Dodgers (21.4 percent).

In simpler terms, Fangraphs suggests that if you simulated the postseason many times, the Cubs would come out as champions less than once out of every five times.

Why are the Dodgers favored over the Cubs? Essentially, they now get to give an extra start to Clayton Kershaw instead of the pitcher in the No. 5 slot in their rotation.

Arrieta hasn’t been sharp, wasn’t sharp last October

In his start on Aug. 18, Jake Arrieta walked a career-high seven batters. Since the beginning of that game, he has a 4.74 ERA (eight starts total).

It’s worth noting that Arrieta struggled in last year’s postseason after shutting out the Pirates in the NL wild-card game. In two starts against the St. Louis Cardinals and New York Mets, he allowed eight runs in 10⅔ innings.

The bridge to Chapman could fall apart

Since he joined the Cubs, Aroldis Chapman has a 1.01 ERA in 28 appearances. He didn’t allow a run in any of his nine appearances after Sept. 1.

So, the ninth inning is no longer a concern for the Cubs. However, getting from their strong rotation to Chapman could be a challenge.

When Cubs relievers pitched in the sixth, seventh and eighth innings this season, they had a 3.84 ERA, 14th in baseball.

Hector Rondon has not been the same since he returned from injury on Sept. 6. Before his triceps injury, he had a 2.47 ERA in 43⅔ innings (12 earned runs total). Since returning, he has allowed eight earned runs in 7⅓ innings.

Heyward could be at the plate in big spots

The Cubs have a big hole in their offense in Jason Heyward. Midway through the season, he was moved from the second spot to the sixth spot in the batting order, from where he has produced a .669 OPS this season.

The sixth spot is important in the Cubs' lineup. Their 1-through-5 spots have produced a major-league-best .371 on-base percentage. Getting those top five hitters on base won't mean much if Heyward can't drive them in.