In his rookie season, Pete Alonso has already hit 27 home runs, second in baseball behind Christian Yelich. He’s sporting a .281/.371/.634 slash line that’s created a 161 wRC+, which trails only Cody Bellinger, Mike Trout, and Yelich. Even after adjusting for defense, Alonso’s 3.3 WAR is seventh among all position players. With numbers like that, it might be difficult to believe that Alonso has already gone through a prolonged slump with just half the season finished. He has, though. Behold:

Peter Alonso’s Slump PA HR BB% K% BABIP ISO wRC+ 4/28-5/28 109 8 4.6% 31.2% .203 .303 95

Hitting like a slightly below-average player for a month might not be classified as a slump for most, but Alonso has been one of the most productive hitters in the game, and the middle-third of his season thus far is a departure from the rest of his numbers. I could have massaged the numbers to make them slightly worse by going from April 30 to May 25, but the above sample fits neatly as the middle third of Alonso’s season so far. Earlier this season when Alonso was doing very well, Ben Clemens discussed the good and bad aspects of the first baseman’s game, though Ben’s focus was on the good as Alonso was annihilating baseballs at the time:

The two parts of this article are the Pete Alonso experience in a nutshell. The quality of contact isn’t the question — when Alonso hits something, it goes a long way. The question is always going to be whether he can make enough contact to tap into his tremendous raw power. The early returns are promising, but they’re also confusing. Alonso can hit — not that there were many questions about that before — but he’s been a pure manifestation of 80-grade power this year. He’s also struck out 30% of the time, which is, you know, not great.

What we saw from Alonso for much of May was the troublesome aspect of his profile. He was swinging and missing a lot, and not drawing walks. He was hitting home runs, but he produced such a low batting average that his hitting line was below average. We could chalk the BABIP up to bad luck, but his expected numbers from Statcast weren’t much better than what he actually produced. For a first baseman with potential fielding issues, that profile is nearly unplayable on a contending team. Alonso was experiencing his first adjustment from pitchers, and for about a month, he adjusted poorly.

Here are Alonso’s plate discipline numbers from the first few months of the season:

Peter Alonso’s Plate Discipline Changes O-Swing% Z-Swing% Swing% O-Contact% Z-Contact% Contact% Zone% F-Strike% SwStr% 3/28-4/27 33.0 % 63.4 % 44.8 % 58.8 % 83.1 % 72.1 % 38.8 % 59.6 % 12.5 % 4/28-5/28 38.6 % 69.8 % 49.5 % 68.9 % 83.7 % 76.2 % 35.1 % 56.9 % 11.8 %

Pitchers threw to Alonso more carefully and he responded in the worst way possible: swinging more. His O-Swing% numbers were already below average at the start of the season, and for a month, he got worse. In visual form, here’s Alonso’s swing percentage heatmaps from those periods:



Maybe those maps don’t look completely different, but look at the sections up and in, and low and away. On the left map from April, Alonso isn’t swinging frequently at those pitches, while in May, he offered at the same pitches more often. That he made contact more often on pitches out of the zone might have prevented even more strikeouts, but it also caused weaker contact when Alonso put the ball in play. At the height of his slump, he told Mike Puma of the New York Post about his need to focus on pitch location.

“If I am swinging at junk then they are going to keep throwing it,” he said. “If I am doing my job and swinging at strikes and taking borderline pitches or sitting on good pitchers’ pitches and being locked in on my zone then it is kind of rewarding if I get a quality pitch to hit, whether it be a breaking ball that’s over the middle of the plate or changeup over the middle or fastball. “For me it doesn’t matter what type of pitch it is, I need something middle, because even if I expand the zone on a fastball then I am still going to get myself out or not get myself in a good count to do damage or help the team.”

Alonso identified and understood his weakness. Making a demonstrable change against the best pitchers in the world can be difficult. Here’s his swing map over roughly the last month:

That area down and in is still the place to throw it if you want to try to get it by Alonso, but he’s pretty clearly swung less in an effort to avoid chasing bad pitches. That up and in area, and the low and away area look even better than they did the first month of the season. The lack of swings on the outside of the zone isn’t an indication it is a safe place to throw there, though, as Cole Hamels recently found out:

This is what Alonso’s plate discipline numbers have looked like over the past month compared to the first two we saw above:

Peter Alonso’s Plate Discipline O-Swing% Z-Swing% Swing% O-Contact% Z-Contact% Contact% Zone% F-Strike% SwStr% 3/28-4/27 33.0 % 63.4 % 44.8 % 58.8 % 83.1 % 72.1 % 38.8 % 59.6 % 12.5 % 4/28-5/28 38.6 % 69.8 % 49.5 % 68.9 % 83.7 % 76.2 % 35.1 % 56.9 % 11.8 % 5/29-6/25 29.8 % 59.9 % 39.9 % 66.3 % 84.1 % 75.3 % 33.7 % 59.7 % 9.9 %

I highlighted the O-Swing% and Zone% because those stats show the changes pitchers have made against Alonso and how Alonso has responded. Pitchers continue to hope Alonso chases, and are hopeful to avoid the strike zone. The Mets’ slugger initially responded with poor plate discipline but has since responded with fewer swings. As a result, Alonso has been better lately than he was in April when he took the league by storm:

Peter Alonso’s Big Month PA HR BB% K% BABIP ISO wRC+ 3/28-4/27 109 9 11.9% 27.5% .364 .372 182 4/28-5/28 109 8 4.6% 31.2% .203 .303 95 5/29-6/25 114 10 13.2% 17.5% .328 .394 204

Alonso has seen a massive decrease in his strikeout rate as pitchers have opted to avoid the strike zone. It’s possible his passivity could be exploited, but that’s a dangerous game given his ability to punish strikes. For good measure, here’s how his Statcast statistics during the same time periods measure up:

Peter Alonso’s Statcast Ranks xwOBA MLB Rank (75 PA) xwOBACON MLB Rank (50 BB) 3/28-4/27 .407 18 .530 10 4/28-5/28 .336 107 .464 34 5/29-6/25 .472 2 .539 3 SOURCE: Baseball Savant

Since the end of May, only Mike Trout and Jorge Soler have been hitting the ball with more authority when contact is made. Factoring in walks and strikeouts, only Trout has exceeded Alonso by Statcast’s metrics over that span. Statcast’s numbers align with our own, as Mike Trout has a 227 wRC+ since May 29, with Alonso second to Trout with a 204 mark just besting Christian Yelich. He’s made some of my skepticism at his pace seem foolish, but at least I’m in good company along with a lot of the game’s best pitchers.