Over the years, the Cubs have had some very good or even great pitchers on their staff. Hall of Famers like Fergie Jenkins and Greg Maddux, guys with potential who never made it like Kerry Wood and Mark Prior, guys who had great career starts and fizzled like Carlos Zambrano.
Nothing in Jake Arrieta's history would have you think he could put together one of the best pitching seasons in Cubs history, but he is heading that way, at least in the post-World War II era.
Here are all the Cubs pitchers since 1945 who have posted an ERA as low or lower than Arrieta's current 2.52 over a full season (qualified for the ERA title):
That's a pretty short list. Bill Hands had the best year of his career in 1969, posting 8.4 bWAR, and it still wasn't enough to get the Cubs to the playoffs. Dick Ellsworth would have won the Cy Young Award in a year like that one if there hadn't been another lefthander around (Sandy Koufax) who was even better.
Granted, ERA is not the only nor the best criteria to rank a pitcher, and with so many National League starters having excellent seasons, Arrieta's ranks just eighth. But over his last six starts his ERA is 0.96 in 46⅔ innings (five earned runs), with a WHIP of 0.664 and 44 strikeouts, and no home runs allowed. Batters are hitting .152 against Jake in that span. These numbers nearly match Zack Greinke over his last six starts, where Greinke hasn't allowed a run at all. And I still think Arrieta has a no-hitter somewhere in his future.
Individual pitcher wins aren't that important in modern baseball, but it should be noted that Arrieta is on pace for a 20-win season, something that's been done only once by a Cub since 1992 (Jon Lieber, 2001) and just four times since 1971, which was the last year a Cubs pitcher won more than 20 games (Fergie, 24 wins in his Cy Young season).
The Cubs signed Jon Lester to be the ace of their staff, but they already had one in Arrieta, who leads one of the best rotations in the National League. Arrieta's 2015 season could rate as one of the best in modern Cubs history.