The Cubs are filled with question marks. The biggest currently on the pitching staff is probably Jake Arrieta. The former Oriole performed extremely well for the Cubs in terms of results after the change of scenery trade. The 3.66 ERA while with the Cubs is well below Arrieta's Oriole career ERA of 5.46, but unfortunately doesn't represent something that is likely sustainable. His BABIP with the Cubs was a ridiculous low of .193. Arrieta will get a chance to prove if he can take that step forward to being a starting pitcher at the start of this season, but what should be our expectations for this talented but flawed starter.
The reason Arrieta was once a top 100 prospect and opening day starter is the quality of his stuff. After the trade deadline Phil Rogers gave us the quote from AJ Ellis about Arrieta's stuff:
The stuff he threw up at me was stuff I haven’t seen all year – the sharpness of his pitches, the way his fastball darted out of the zone.
And yet Arrieta's strikeout rates and swinging strike rates (a strong statistical indicator of strikeout rates) are more Travis Wood than Jeff Samardzija. That seems odd that someone with Arrieta's quality of stuff can't get more swinging strikes and therefore more strikeouts, but an interesting piece of information might come from spin data. There have been a series of articles out there that suggest the number of rotations a pitcher can put on their fastball effect the swinging strike and groundball rate far more than velocity. That article had the following chart:
Now Arrieta's fastball the past two seasons with the Orioles checked in at 2216 rpm for his fourseam fastball and 2362 rpm for his sinker according to Texas Leaguers. Those rates would place him on the chart with somewhere between a 7.7% and 8.7%. Compare that to Clayton Kershaw's 2452 rpm or Jeff Samardzija's 2434 and it is a bit easier to see why Arrieta's stuff might not translate into big strikeout numbers despite lighting up the radar gun at times.
An interesting thing happened when Arrieta pitched in Chicago though. He began use the sinker far more than the four seam fastball and his rpm went down by almost 100 rotations on both pitches. This would match his significant increase in groudballs per plate appearance. His groundball percent jumped from 18.25% to 23.68%. This is a small sample size to be sure with the Cubs, but Chris Bosio has been known to be groundball magic as well. I'm willing to buy that this could be a potential long term trend and not a sample size aberration.
Arrieta has massive control issues, and that has been the largest problem he has faced. He walks too many guys, and often falls behind to hitters counts which forces him to give up more hard hit balls. His walk rate remained high while with the Cubs, and unfortunately there isn't much hope in this regard. Two slivers of hope is an increase in first strike percentage last year from 56.5% to 59.6% and a high infield fly ball percentage while with the Cubs which indicates weaker contact.
Arrieta also has had issues with left handed batters and with runners on base. His numbers in those situations have largely been bad. He fared better against left handed hitting with the Cubs, but again mostly driven by an unsustainable low BABIP. Arrieta has some annoying similarities to Chris Volstad in the regard that he might always be susceptible to the big inning and therefore consistently underperform his peripherals. The only hope I have in that regard is that pitchers are far more capable of breakthroughs in performance later in life than hitters. The Cubs have had several pitchers radically improve at around the same age as Arrieta in recent memory (Jeff Samardzija, Ryan Dempster, Matt Clement), and there are a few pieces of information for which to hang an appropriately small amount of hope on.
Jake Arrieta is perhaps the biggest x-factor in the Cubs pitching staff next season. His performance could range from a guy that is forced into the bullpen or DFA all the way to becoming a competent big league starting pitcher with flashes of greatness with his stuff. There isn't much right now to predict that he will take that step forward at the moment. However, an offseason with the Cubs staff and change of scenery might bring a bit of luck that allows Arrieta to reach that promise he showed just a few years ago in the Orioles minors. On a team full of questions, Arrieta being an answer would go a long way towards bring the future into clarity.