We don't necessarily have to revisit Gary Sanchez’s dominance last season, but he was so good that there's no harm in looking at his awesome numbers again. After being called up in August, Sanchez at his best turned in what was essentially one of the hottest hitting streaks of all time. In totality, after the Yankees called him up on August 3rd, he batted .305/.382/.670 with 20 home runs and a 176 wRC+.
Those numbers do include, however, a relative slump. If we split Sanchez's season into August and September/October, his numbers look like this: a .389/.458/.832 line with a 240 wRC+ in August, versus a .225/.314/.520 line and a 119 wRC+ in September/October. The latter figures are still good, but not nearly as Herculean as Sanchez's initial run in August.
Is there anything to be gleaned from Sanchez's "slump"? Were there any hints as to how pitchers intend to attack his weaknesses in the future, any glimpse into the holes in Sanchez's game? Let's take a deeper look.
First, let's try to see if opposing pitchers adjusted to Sanchez's Ruthian beginning. After watching him terrorize big league pitching for a full month, it would seem intuitive that opponents might try to attack Sanchez differently, perhaps throwing fewer fastballs in the zone and challenging him less.
According to Brooks Baseball, there is a bit of evidence for that. In August, over 55% of the pitches Sanchez saw were fastballs. That figure was down to 52% in September, while the share of offspeed pitches he saw increased from 10% to 15%. So pitchers may have been more reticent to throw Sanchez hard stuff, but not enough to amount to a huge change.
Were opposing pitchers more hesitant to simply throw pitches in the strike zone, once they saw Sanchez's power on display? Again, the evidence is limited. Per FanGraphs' plate discipline data, 40.4% of the pitches Sanchez saw were in the zone during August, which dipped to 39.1% after August. Another slight change worth noting, but nothing ground-breaking.
Pitchers didn't change their approach to Sanchez too much as the season wore on, so it's fair to guess that much of Sanchez's slip was simply on his part. Given how incredible Sanchez was initially, it's only natural that he would fall off on his own. During Sanchez's come down, some of his weaknesses that were noted as he came through the minors did seem apparent.
Scouting reports on Sanchez used to glow about his power, but were sometimes critical about his approach at the plate and his tendency to expand the zone, with some concern about how much of his power would be held back by his pure hit tool. Some of those concerns looked justified down the stretch. While Sanchez didn't swing more in September than he did in August, the rate at which he swung at pitches in the zone fell, and the rate at which he offered at balls increased. His out-of-zone swing rate of 34.0% in September was above the league average of 30.7%, and his in-zone swing rate of 62% was below the league average of 67.0%
It's then not too surprising that Sanchez's strikeout rate spiked, from 19.6% in August to 29.7% onward. In 2017 and beyond, it seems possible that Sanchez's tendency to swing at pitches out of the zone could limit his ability to approach the ceiling he demonstrated last summer.
Otherwise, most of Sanchez's downturn can probably be attributed to some drop in performance and luck. Most notably, after lambasting fastballs when he first came up, Sanchez's performance against hard pitches seemed to suffer. The rate at which he whiffed and struck groundballs against fastballs increased in September, and his batted ball speed against hard pitches fell from over 95 mph in August to 92 mph afterward. After batting .333 against four-seamers and .417 against sinkers in August, Sanchez hit .172 and .278 against those pitches, respectively, in September.
However, worse luck in September probably also hurt Sanchez. He was never going to sustain a BABIP over .400, but his .233 BABIP after August does him a disservice, as he didn't strike the ball with much less authority during the season's final month. As mentioned above, his batted ball speed on fastballs dipped, but he saw slight increases on batted ball speed against offspeed and breaking stuff. To visualize, here is a look at Sanchez's batted ball exit velocity over time, courtesy of Baseball Savant:
There is a perceptible drop at the end of August and beginning of September, but overall, Sanchez was basically scalding the ball constantly. Even at the trough in the graph, Sanchez's average exit velocity beat that of the league as a whole.
Even at his worst, Sanchez was good, which underscores the most important thing about Sanchez's slump: he was still quite valuable. A catcher that posts a 119 wRC+ with quality work in the field is an extremely worthwhile player, one capable of finishing second in fWAR among catchers in the season’s final month. Based on Sanchez's September, there was only a little bit to learn about how pitchers adjusted to him and how Sanchez's weakness might hamper him. The main takeaway, perhaps, is that even as he struggled, Sanchez looked pretty great, providing the Yankees plenty of hope that their young backstop will be a star.