Why is Rick Porcello giving up so many home runs?

Although he’s been far from the biggest problem on this disappointing 2015 Red Sox team, Rick Porcello has definitely been inconsistent. The good thing is he’s shown a few flashes of the pitcher that he can be, specifically the four-start stretch in late April and early May when he pitched to a 2.10 ERA. With that being said, he’s also had some horrible starts. Put it all together and you have a guy who’s sporting a 5.37 ERA (75 ERA+) with a 4.83 FIP. If you’ve been watching, it hasn’t been hard to see what the biggest issue has been. Porcello’s been giving up homers at far too highly of a rate.

While it’s a bit counterintuitive to think a ground-ball pitcher like Porcello would be prone to giving up home runs, this is not exactly a new issue for him. Of course, he’s never been plagued to this degree, but he had been giving up just about one home run every nine innings coming into this season. While that’s by no means terrible — it's been effectively league-average — it’s not what you want out of a ground ball pitcher who doesn’t strike many batters out.

This year, the issue has gone from an annoying but endurable part of his game to a severe problem. As many of these type of early-season issues seem to be, Porcello’s dinger problem can be at least partially blamed on bad luck. He’s up to just about a 15 percent home run to fly ball ratio, more than four percentage points higher than the league-average. However, he’s been at or above league-average in this area in every year of his major-league career, so we can’t completely write it off as bad luck.

The issue in 2015 has been a huge increase of balls in the air, specifically fly balls. Per Fangraphs’ batted ball data, his fly ball percentage is up to 38 percent, ten percentage points above his career-average and six above his previous career high. Different sites disagree about what’s a line drive and what’s a fly ball, but that’s not really important here. The issue has been his declining ground ball rate. Both Fangraphs and Baseball Prospectus have his GB% falling down to 43 percent from around 50 percent last year and well above that mark earlier in his career. Porcello allows non-grounders to go as homers more than the average pitcher, making it essential for him to keep the ball on the ground.

When he is allowing non-grounders, opponents are crushing the ball this year. Looking once again at his Fangraphs player page, their new hard-hit metric Hard% has him allowing hard contact on roughly a third of allowed balls in play. For context, that is the 14th highest mark among the 101 pitchers who have thrown at least 50 innings. On top of that, Baseball-Reference has his line drive rate sitting at a far-too-high 30 percent line drive rate.

So, Porcello has given up a lot of balls in the air, and they’ve generally been hit hard. That’s not really new information, but it is important to understand when looking at his homer problems. The next step is trying to figure out why. To do this, the first thing to look at is his zone profile. There’s a pretty glaring issue there right off the bat.

The ball is being left in the upper portion of the strike zone far too often in 2015 (the second picture). Previously in his career, he’s pounded the bottom of the zone, which of course was the major reason he’s induced so many ground balls. So, leaving the ball up in the zone is the biggest problem, right? Well, it may be an issue, but surprisingly opponents haven’t really been punishing these pitches. Check out his opponents’ Isolated Power for each section of the zone below.

That’s a lot of damage being done in the bottom part of the strike zone, especially on the arm side. This lead me to believe there has been a repertoire change that may be contributing to this, and it turns out I was (somewhat) correct.

Unsurprisingly, Porcello has relied on his sinker a lot in the past, throwing it almost 50 percent of the time from the start of his career through last season, per Brooks Baseball. This season, he's scrapped that strategy. He’s still throwing the pitch, but he’s knocked his usage down to 31 percent. To make up for it, he’s throwing his four-seam fastball more, up to 34 percent of the time. While this change has resulted in more strikeouts — Porcello is striking out more than seven batters per nine innings for just the second time in his career — it’s resulting in more damage as well. This season, opponents have a .175 ISO against his fastball versus a .093 mark against the sinker.

Additionally, he’s essentially cut the slider from his repertoire and has eased up on his changeup usage. The latter has actually been a positive, as batters have an absurd .483 ISO off the pitch this season. To make up for these reduced usages, he’s thrown many more curveballs and added a cutter into the mix. Again, this is helping him with the strikeouts, as both pitches result in more swings and misses than his change up and slider did, but they’re hurting him overall. Opponents have an ISO of .200 and .231 against his cutter and curveball, respectively. Generally speaking, his attempt to induce more strikeouts has proven successful, but it’s hurting him in other areas.

Porcello hasn’t been the pitcher many had hoped he’d be in the early part of the season. The good news is there is surely some bad luck here. The bad news is, even with the luck, there are a lot of bad signs. He’s getting hit hard while simultaneously allowing fewer grounders; a recipe for disaster. While the balls he’s leaving up the zone haven’t really hurt him too badly just yet, they surely will sooner or later. The solution may be going back to his old style, relying less on strikeouts and more on ground balls. This would entail returning back to his heavy sinker usage while also relying less on curveballs and cutters. It’s far too early to write Porcello off for this season and the future, but the home runs have been discouraging and he needs to make the proper adjustments.