Red Sox slugger Mike Napoli is back

Mike Napoli has long been a streaky hitter. Any Red Sox fan who has watched Napoli hit the past couple seasons knows the slugger's home runs often come in bunches, as they did this past weekend when Napoli hit four home runs in a three-day span.

Yet leading up to his power surge, the first baseman was in the midst of a prolonged slump that, in many ways, seemed to defy explanation, even for someone as streaky as Napoli. Prior to last Friday's contest, he was batting just .178/.274/.310 through 146 plate appearances. Although his .209 batting average on balls in play gave hints as to some of those struggles, the disappearance of Napoli's power was the real head-scratcher.

That all changed when he punished Angels pitching yet again over the weekend. Napoli sent three balls over the Green Monster and another deep into the center field bleachers, reminding everyone how formidable his bat can be when he is performing well at the plate.

The reasons behind Napoli's early-season struggles and his subsequent breakout are a little less clear. The 33-year-old certainly wasn't having much luck on balls in play, but he's also seen a noticeable dip in his line-drive rate over the past three years, from 24 percent back in 2013 to 19 percent a season ago to just 14 percent this campaign. That would be his lowest mark since his rookie season, so perhaps Napoli was simply hitting the ball with less authority than he has in the past.

According to Baseball Savant, though, Napoli has the 20^th-highest batted-ball velocity among all MLB hitters, which doesn't exactly jive with the notion that he is suddenly making weaker contact. That Napoli's scuffles came over a sample of under 200 plate appearances makes it even harder to draw much in the way of meaningful conclusions.

The one thing we do know about Napoli is that he feasts on mistake pitches, with much of his power coming against offerings left up in the zone and over the plate. Here is a zone chart of Napoli's slugging percentage in 2013 and 2014, which demonstrates the types of pitches he has the most success against:

This was probably the most noteworthy part of Napoli's four-homer outburst, as he punished the Angels' hurlers for missing their spots and leaving their pitches up in the zone.

The other trend that stands out about Napoli's recent hot streak is that he's pulling the ball with authority again. According to FanGraphs, Napoli's overall pull percentage has dipped to 35 percent this season, falling eight percentage points from his 2014 mark of 43 percent and standing well below his career average of 44 percent.

Just comparing his spray charts over the past three seasons shows how much less Napoli has pulled the ball in 2015.

In 2013-14:

Source: FanGraphs

Source: FanGraphs

Even though he's shown plenty of power to center and right field in the past, the majority of Napoli's homers have always come on balls he drives to left.

All this makes his sudden power surge even more encouraging. Napoli is again punishing pitchers for their mistakes and again pulling the ball in the air. Why he was failing to do so for the better part of two months is a bit of a mystery.

Napoli spent time with Dustin Pedroia looking at video and discussing his mechanics during the club's flight back from Seattle two weekends ago, and perhaps the two picked up on some adjustments the slugger has since put to good use. He did tell Gordon Edes of ESPN Boston that he "wasn't getting into [his] position," in time at the plate, and while that might sound overly simplistic, sometimes minor fixes can make all the difference for power hitters like Napoli.

Jose Bautista, for example, credited his power breakout in 2010 with the simple adjustment of starting his stride earlier, and the same principle could hold true for Napoli as well. Starting his swing a moment or two earlier might just be the difference between Napoli pulling mistake pitches with authority and flying out meekly to the center fielder (or swinging through a fastball up in the zone).

Whatever the reason, Napoli currently resembles the type of middle-of-the-order threat the Red Sox offense could use at the moment. With David Ortiz and Hanley Ramirez both slumping, Napoli's outburst at the plate couldn't have come at a better time.

How long he continues this hot stretch remains to be seen. For now, at least, Napoli is performing like his very best self, jumping on pitches left out over the plate and driving them deep to left field.