If a towering home run off Rick Porcello on just the second pitch of today’s spring training game against Baltimore felt disconcerting, that’s because it should be.

A strained relationship between the former Detroit pitcher and Red Sox nation formed before Porcello ever threw a pitch in a Boston uniform. Even for a pitcher who had shown strides as Tiger the last couple of seasons, a substantial four-year contract at 82.5 million dollars did not seem like surefire positive move. Following that, Porcello underperformed, throwing his tenure in Boston into immediate question and greater scrutiny.

The notion of him having a poor season became very apparent in traditional stats, posting a 9-15 win-loss record and tied his worst single-season ERA mark of 4.92–far above his 4.39 career average and 3.43 ERA the prior year. Usually, these kind of stats, though most widely used, prove deceptive and misleading. Yet even with the more reliable fielding independent pitching metric, Porcello clearly had a poor pitching season: his 4.13 2015 FIP fell above his 4.04 average and above his three consecutive sub-4.00 FIP seasons beforehand. Capturing it all, Porcello experienced a negative 42.9 percent change in his WAR from 2015 to 2015.

A closer look at all his peripheral stats easily point to troubles with home run yield rate–and the characteristics of hits that lead to home runs–as the downfall of Porcello’s season in 2015. After all, he did give up his highest home run total in a single year with 25 in 2015, nearly six more than his career average.

**Home run problems**

As seen above in measuring his home run per nine inning rates across his career relative to a career average of 0.98, Porcello had his worst HR/9 rate of his entire life as an MLB starter in 2015 with the Red Sox (higher totals means more home runs yielded, a worse outcome). Moreover, his first season in Boston marked the largest absolute change in year-to-year HR/9 in his career as well, as his HR/9 swung greatly for the worse from 2014 to 2015.

Doing the same analysis for Porcello’s home run to flyball ratio–comparing with a 11.8 career average–offers a very similar picture. The pitcher gave up the most home runs as a percentage of his flyballs yielded during his seven-year career in 2015, though this most recent stat diverges less from previous single-season marks than does his 2015 HR/9 ratio.

Crucially, xFIP tells a very different story. This stat measures expected FIP and calculates it the same way as FIP except that home runs are 10.5 percent of fly balls induced–it estimates how many home runs should have been allowed in terms of league average home run rate. Porcello posted an xFIP of 3.72, just above his 3.68 mark the year before but much better than his career average of 3.85. In other words, this metric and comparison helps isolate Porcello’s propensity to yielding home runs in 2015 as the source for his broader, lackluster pitching.

**Contact on pitches**

The type of hits Porcello allowed also lend insight into his struggles. Harder hit balls not only more often fall for hits and extra bases, but also more likely end in home runs. Interestingly, Porcello allowed many more “hard-hit” balls–as opposed to medium or soft ones–in 2015. As seen in the above graph and table (with the key points towards the right hand side showing the changes in the 2015 season), Porcello experienced a 5.6 percentage point increase in the percentage of all batted balls that were hard-hit, the third largest year-to-year change among all three of the categories. This, along with the fact that it stands considerably above his career average of 26.7 percent, undoubtedly contributed to his home run woes; batters were making much harder contact with Porcello’s pitches than ever before.

Batting average on balls in play, the stat that notoriously fluctuates heavily and over which pitchers have little control, might explain some of Porcello’s struggles, but not many of them. At .332, his BABIP was above a .313 career average. This should indicate slightly worse luck with balls in play and defense behind him, but without a more sizable discrepancy between career and single-season numbers here, BABIP does not hold too much explanatory power.

**Home ballpark effects**

His decline also has little to do with the environment in which Porcello pitched. Across all MLB stadiums, some are more favorable to hitters than others, a notion quantified by park factors. Here’s the park factor rank in which Porcello pitched for the last seven years in terms of how favorable it was for hitters’ home run totals:

Given that a lower rank is more favorable to the hitter, there very clearly exists little difference in playing in Boston than playing Detroit for Porcello, as it pertains to one ballpark being more conducive to home runs for batters than another. If anything, Boston should have proved more advantageous in this respect.

**Differences by pitch type**

In terms of differences in pitch type, a few changes in Porcello’s pitching repertoire might have contributed to his struggles.

As seen in the above graph that compares pitch type usage in 2015 to his career averages for it, Porcello concentrated less on his sinker and more on his fourseam, cutter, and curveball. What’s more interesting is what type of hitting power–proxied by the isolated power (ISO) metric–opposing batters put up against his pitches, as shown below in this graph from BrooksBaseball:

Though using it at the same rate as in the past, Porcello yielded a much higher ISO to opposing batters on his changeup than before. This change marks the greatest one along his transition from Detroit to Boston, and thus holds the greatest significance in terms of the role of pitch type dynamics in Porcellos’ 2015 season.

**Long ball issues, but strikeout improvement**

The centrality of long ball struggles explains why several other key peripherals–not related to home runs allowed–did not prove indicative of a broader decline despite Porcello performing poorly overall. Though his walk rate did increase somewhat (but still remained below his career average), Porcello in fact put up the best strikeout numbers of his career: relative to both 2014 and his career average, Porcello experienced surges in strikeouts per nine innings, strikeout percentage, K/BB ratio, posting career-best across all these metrics in 2015. The importance of these underlying measures–of which home runs are a part–derives from their strong predictive ability for future success. So while declines in home run rates may herald future concern for Porcello in Boston, his numbers pertaining to strikeout rates signal improvement in the near future.

Accordingly, an average of projection systems from FanGraphs (under “Depth Charts”) predicts much better results for Porcello in 2016–namely, a 0.8 WAR increase to 2.4 from 2015, and importantly a 0.96 HR/9 mark, more in line with his career average and much better than his 1.31 ratio in 2015. In 2016 more than ever, Porcello’s ability to limit home runs hit off his pitches will heavily shape his performance on the mound.

*All stats from FanGraphs.com unless otherwise noted.*