Is Arrieta the Cubs’ True Ace?

So we all know the Cubs signed Jon Lester to a six-year, $155 million dollar contract this offseason. The Cubs presumably believe they will be competitive if not this season then the next, and therefore decided to get themselves an ace. This however bodes the question, is Jon Lester even the Cubs’ best pitcher going into 2015?

Last year proved to be a breakout year for right-hander Jake Arrieta. Arrieta was drafted in 2007 by the Baltimore Orioles and made his Major League debut in 2010. He spent a little over six years with the Orioles before he was traded to the Cubs in 2013. Arrieta posted good numbers in the minors, in fact in 2010, at Triple A he had a 1.85 ERA before getting the call to the Majors that same season. In the Majors, however it was a different story. From 2010-2013 Arrieta was downright awful, never pitching more than 119.1 innings in a season and never posting an ERA below 4.66, which he did in his rookie year.

2014, though, was different. Arrieta posted the best numbers of his career, finishing with a 2.53 ERA, a 2.26 FIP, and a 2.73 xFIP. He also recorded a career high in innings, netting 156.2 innings pitched. How was Arrieta able to this? A guy who had never had an ERA below 4.66 recorded a Cy Young-caliber season? He even might have had a shot at the Cy Young Award if he’d pitched more innings.

Well Arrieta essentially stopped walking hitters and started striking out a bunch of hitters. He posted the best K-BB% of his career at 20.5% and he also stopped giving up home runs at .29 HR/9. There are several ways a pitcher can become better; some of them create a new pitch, some of them make a mechanical adjustment, and some just sequence their pitches better. I think in Arrieta’s case it comes down to sequencing and maybe mechanical although I have no way of truly knowing whether the latter is true or not.

Here is an example of the type of pitches Arrieta threw from 2010-2013 according to Brooks Baseball.

2010-2013 Fourseam Sinker Slider Curve Change
LHH 27% 33% 9% 19% 13%
RHH 32% 31% 24% 11% 1%


Here is Arrieta’s sequencing in 2014.

2014 Fourseam Sinker Slider Curve Change
LHH 19% 24% 26% 21% 10%
RHH 21% 31% 32% 14% 1%


Two elements really stand out to me through these tables. The first is that Arrieta has not added a killer new pitch. The second is that Arrieta is throwing a lot less four-seam fastballs and a lot more sliders, especially to left-handed hitters. He’s also increased his curveball usage. Arrieta essentially is mixing his pitches a lot more than in previous seasons, which could be an answer to his sudden spike in production. If you’re thinking, well, maybe he’s throwing harder, he’s not. His fastball velocity last year was 93.4, which is pretty much where it’s been its entire career (career fastball velocity: 93).

Does this guarantee that Arrieta will be better than Lester next season? Probably not. Lester still has Arrieta by a wide margin in innings. Lester’s consistently pitched more 200 innings throughout his career, while Arrieta’s never pitched more than 156.2. Also even though Arrieta is mixing his pitches better, this isn’t necessarily predictive that he will keep doing it or keep doing it with the same success rate. If I personally had to put money on it I would still give a slight edge to Lester. That being said I wouldn’t be surprised if Arrieta was better than Lester next season and going forward.

Arrieta at 28 is still three years younger than Lester (31). While Arrieta’s fastball velocity had kept steady, Lester’s fastball velocity has been on a steady downward decline since 2010. Last year his fastball velocity was the lowest of his career at 91.5 and if it keeps dropping we could see a significant decline in Lester’s production. Throughout his career, Lester’s ERA and peripheral indicators have consistently been in the mid- to low-threes. It wouldn’t surprise me if Lester fell back to that norm, or even took a step back.

Essentially, it is difficult to predict which pitcher will regress and which one will keep the same level of production. For all we know, they could both regress. The point here is to demonstrate that Lester will not necessarily be that much better (if better at all) than Arrieta in 2015. For all we know, Arrieta might be the next Cubs ace.

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Julien Assouline is a journalism student at Ryerson University, and writes for Baseball Prospectus Milwaukee. Follow him on Twitter @JulienAssouline.

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“although I have no way of truly knowing whether the ladder is true or not.”

You mean “latter” not “ladder”.

Please feel free to delete this comment after making the correction. :)