​4 Starting Pitchers Due for Regression in 2016

Which pitchers are likely to take a step back in 2016 based on the advanced numbers?

Every season, there are bound to be breakout players who post career-best numbers. Generally, these breakouts are a result of an improvement in skill and/or opportunity.

In some instances, a player can benefit from abnormal amounts of luck -- resulting in statistics that don't match the true performance of the player. As a general rule, the stats will regress to the mean over time.

Here's the problem, though. If a player benefits from good luck for the entirety of a Major League season, most fans won't take the time to check out the underlying stats and will assume that the player had a breakout due to an improvement in skill.

Using Skill-Interactive ERA (SIERA) and xFIP, the two most popular ERA estimators, we can discover which pitchers benefited from luck rather than an improvement in skill and predict who will see their stats regress in 2016.

Zack Greinke, Arizona Diamondbacks

Zack Greinke had everything go in his favor last season, as he posted an unsustainable 1.66 ERA on his way to a huge contract from Arizona this offseason. Greinke's ERA was the lowest by a National League pitcher since Greg Maddux posted a 1.63 mark 21 years ago. It's clear that Greinke won't be able to match last season's numbers, but a deeper look at his peripherals uncover just how severely inflated his numbers were.

Greinke's 2015 SIERA of 3.27 and xFIP of 3.22 both suggest that Greinke's 1.66 ERA wasn't anywhere close to representing how he actually pitched last season. The difference between Greinke's ERA and SIERA was the largest in the league. Greinke's strikeout rate (K%) was down, too, from 25.2 percent to 23.7 percent. His fly ball rate was up as well, as his 32.9 percent mark was the highest it's been since 2010.

So, how was Greinke able to put up one of the best statistical performances in recent memory? Basically, everything went right for him. Despite allowing more fly balls, Greinke gave up just 14 homers -- his second-lowest total in the past six seasons. His 7.3 percent home run to fly ball ratio is unsustainably low and a clear indicator of good luck.

Greinke also benefited from a batting average on balls in play (BABIP) of .229, which was the second-lowest in the Majors and significantly lower than his career mark of .305 prior to the 2015 season. He also had the highest strand rate in the league (86.5 percent), which helped him limit runs. His career strand rate of 74 percent prior to 2015 would hint that Greinke is likely due for some regression from his 86.5 percent mark from last season.

Greinke will likely still be among the top 10 starting pitchers in 2016, but it's almost certain that he will not return to his 2015 form.

Jake Arrieta, Chicago Cubs

Jake Arrieta, the man who beat out Greinke for the Cy Young, was almost as lucky as the aforementioned ace. Arrieta posted a 22-6 record, 1.77 ERA, and 0.86 WHIP last season, despite peripheral stats that were pretty similar to his 2014 numbers.

Arrieta's 2015 xFIP of 2.61 and SIERA of 2.75 were nearly a full run ahead of his ERA and were right in line with his 2014 xFIP of 2.73 and SIERA of 2.83.

His 2015 strikeout rate of 27.1 percent was nearly a mirror image of his 2014 mark of 27.2 percent. Arrieta did make big improvements in his ground ball rate and his walk rate, but a lot of the improvement he showed in 2015 can be chalked up to good luck.

One example of such good luck was Arrieta's .246 BABIP, which was the third-lowest mark in the league. Arrieta's BABIP in 97 starts prior to 2015 was .281. Arrieta was also somewhat lucky when it came to stranding base runners, as his 80 percent strand rate ranked fourth-highest in the Majors and much higher than his 68.7 percent career mark prior to 2015.

Arrieta also has more health concerns entering 2016 than most other aces. The 229 innings he logged in 2015 was easily a career high. Before last season, he had been unable to top 170 innings in any season, including minor league innings. Injuries were a big reason it took so long for Arrieta to break out in the first place.

Don't get me wrong, Arrieta is still one of the elite pitching talents in the game right now -- it's just very difficult to envision his repeating his 2015 performance. If you expect another sub-two ERA, you'll likely be disappointed. A more realistic projection is probably somewhere along the lines of an ERA in the mid-twos with 200-plus strikeouts, which is exactly where our projections see him finishing.

Marco Estrada, Toronto Blue Jays

Entering 2015, Marco Estrada had a career ERA 4.23 ERA and had never posted a season with more than seven wins. Last season, he put up a career best 13-8 record and 3.13 ERA, despite peripheral stats that suggest a decline in skills.

A look past Estrada's base stats show that he was possibly the luckiest pitcher in 2015. His xFIP of 4.93 and SIERA of 4.64 indicate that his 3.13 ERA doesn't even come close to representing how he pitched last year. In fact, his xFIP ranked as the third-worst in the league, and he had the largest difference between his xFIP and actual ERA among all qualified starting pitchers.

It's easy to look at the difference between his ERA and ERA predictors and chalk it up to good luck, but let's take a look at the stats just to be sure.

His BABIP of .216 was the lowest in the Majors and significantly lower than his career mark of .276 entering 2015. Estrada also posted a 79.2 percent strand rate, which was the 10th-highest in the league and much higher than his career mark of 71.2 percent.

Estrada also was very fortunate to not give up more home runs in 2015. His fly ball rate of 52.3 percent was the second-highest mark in the league, yet the 24 homers he allowed were only the 21st-most in the Majors. More specifically, Estrada benefited from the 11th-lowest HR/FB ratio in the league. Estrada's 8.7 percent HR/FB ratio was the lowest of his career and is far below the 12.1 percent mark he maintained prior to 2015.

With some regression on balls in play and fly balls, Estrada is almost certain to return to an ERA in the mid-fours in 2016.

To make matters worse, Estrada's strikeout rate dropped for the third consecutive season. His 6.34 strikeouts per nine innings ranked 14th-lowest among 77 qualified starters. Estrada's walk rate went in the wrong direction, too, raising for the third straight year.

Estrada may still be able to build a decent record while pitching for the best offense in baseball, but it's hard to expect him to repeat his 3.13 ERA when you consider what appears to be a deteriorating skill-set.

Yovani Gallardo, Baltimore Orioles

Yovani Gallardo was an advanced statistical anomaly in 2015, ignoring career lows in several crucial stats on his way to a career-best 3.42 ERA. His xFIP of 4.31 and SIERA of 4.59 line up closer with his advanced stats, though, and point toward negative regression in 2016.

Opposing batters were able to hit .267 against Gallardo in 2015, the highest mark he's allowed in his career. He also posted his highest walk rate since 2012. The result was a career-high 1.42 WHIP -- not what you'd expect in the year he posted a career-low ERA.

Gallardo also posted a career-low strikeout rate of 15.3 percent. His strikeout rate has now dropped every season since 2009. Gallardo's K-BB%, or the difference between his strikeout rate and walk rate in 2015, was the third-lowest among qualified pitchers. 

Gallardo's good luck can also be traced in his strand rate and home run to fly ball ratio. His 77.2 percent strand rate was the highest mark he's put up since 2012 and helped him mitigate all of the base runners he allowed. The high strand rate offers some explanation for his career-high WHIP and career-low ERA. Gallardo's 8.8 percent HR/FB was the lowest mark he's put up since 2010, which certainly aided his ERA.

Gallardo left the Rangers for the Orioles this offseason, which doesn't help his win potential. FanGraphs listed Texas as the 5th-best fielding team in 2015 while ranking Baltimore 15th. Texas also finished third in runs scored, while Baltimore was ninth. He'll now have to deal more often with the AL East, which had the 1st, 2nd, 4th, and 25th teams in terms of runs scored in 2015. To make matters worse, no park saw more home runs leave than Gallardo's new home, Camden Yards.

All things considered, it's very hard to envision Gallardo coming close to the numbers he put up last season.