The Chicago Cubs signed Jon Lester to a six-year, $155-million contract in December. Presumably, the Cubs believe they will be competitive, if not this season, then next, and therefore decided to get themselves an ace. That invites a question: Is Jon Lester even the Cubs’ best pitcher going into 2015?

Last season marked 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 organization, before he was traded to the Cubs in 2013. Arrieta posted good numbers in the minors. In fact, in 2010, he had a 1.85 ERA at Triple-A Norfolk before getting the call to the Majors. 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.

His first full season in Chicago 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 do this? How did a guy who had never had an ERA below 4.66 record a Cy Young Award-caliber season?

Well, Arrieta essentially stopped walking hitters, and started striking out a bunch of hitters. He posted the best K-BB% of his career (20.5%), and he also stopped giving up home runs (0.29 per nine innings pitched). 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 shifting use of his repertoire.

Here is a breakdown of the types of pitches Arrieta threw from 2010-2013, according to Brooks Baseball.

Here is Arrieta’s pitch mix in 2014.

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 fewer four-seam fastballs and a lot more sliders, especially to left-handed hitters. He’s also increased his curveball usage. Essentially he’s mixing his pitches a lot more than in previous seasons, which could be the explanation for 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 miles per hour, which is pretty much where it’s been over his entire career (average: 93 MPH).

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 than 200 innings throughout his career, while Arrieta’s never pitched more than 156.2. Also, although Arrieta is mixing his pitches better, this isn’t necessarily predictive—he might slide back into old habits, or he might find the league adjusting to his more blended approach. 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, 28, is still three years younger than Lester. While Arrieta’s fastball velocity has held steady, Lester’s has been on a steady decline since 2010. Last year, Lester’s fastball was the slowest of his career at 91.5 MPH, and if it keeps dropping, we could see a significant decline in Lester’s production. Lester, did put up the best ERA of his career in 2014, but that doesn’t mean he will continue with that same level of 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.