He’s being pitched around at an historic rate.
Pitchers will pitch around hitters for a variety of different reasons. Maybe they’re scared of their power, it could be because the hitter won’t lay off pitches in a certain area outside the zone, it could be because they’re feared.
Since the percentage of tracked pitches inside or outside the zone began in 2007, a common theme developed quickly within the metric. The hitters that saw pitches at the lowest rates inside the zone were either a.) very good, b.) could hit a ball very far, or c.) had a reputation of not letting those pitches go. Since 2007, among 916 hitters with at least 500 plate appearances, common names that fit one of these three tendencies include Joey Gallo (second lowest zone percentage), Javier Baez (fourth lowest), Bryce Harper (fifth lowest), and Vladimir Guerrero Sr. (ninth lowest).
Conversely, on the other side of this list, hitters with the opposite of the three aforementioned tendencies are common. Like Tony Pena Jr. (highest zone percentage), Joey Gathright (fifth highest), and Dave Roberts (10th highest). There’s also a plethora of pitchers filling up the top of list, like Johnny Cueto (second highest).
This all mostly follows common baseball thinking that even a casual fan could understand. If we extend our example-making to just rookies and lower the minimum plate appearance threshold to 100 plate appearances, this still holds true. At the bottom of the zone percentage rankings, we can see that big power hitting prospects such as Pete Alonso (lowest among 952 rookies since 2007), Bryce Harper (10th lowest), and Gallo once again (12 lowest) are there.
As a whole, pitchers generally will pitch to rookies more often than to veterans. This makes sense because, the league-average rookie typically hits around 15 percent below league-average. This makes what about to dive into all the more interesting.
At the bottom of the zone percentage leaderboards (minimum 50 plate appearances) is Vladimir Guerrero Jr., rookie. His 31.6 percent zone percentage is by far the lowest among the 334 hitters qualified. It’s not just that, but it’s also the lowest among 6,991 players with at least 50 plate appearances since 2007. The next lowest rate for this season is Jorge Alfaro, standing at 34.0 percent. The difference between those two is greater than the difference between Alfaro and the sixth lowest, Willson Contreras. Since the metric was tracked, the second lowest rate in a season belongs to 2012 Pablo Sandoval, standing at 32.8 percent. The difference between him and Guerrero is the same as the difference between him and the sixth lowest rate in a season, which would belong to 2019 Alfaro.
Now what does this mean? To be honest, I don’t have a firm grasp on what it does. It probably means that pitchers were scared of Guerrero from the get go (which is an amazing feat for a rookie) and that they still are, despite the slow start. It could mean that his obvious scouting report picked up from Triple A to the majors, it’s just that his side of performance hasn’t correlated from the perfectly from the minors to the majors (yet).
Some great research from Eli Ben-Porat at The Hardball Times shows that how hitters are pitched to in Triple A correlates well to how they are pitch to in the majors. We could probably expect the same to some extent for Guerrero.
With data unavailable regarding Guerrero’s performance inside the zone in Triple-A, the clear expectation is that he destroyed, considering his overall performance (183 wRC+). It could just be as simple as him not getting in the right rhythm since his call-up.
Ironically, Guerrero hasn’t been performing well on pitches inside the zone. His in-zone xwOBA ranks 345th out of 365 qualified major league hitters. I think these issues can be pinned to his swing, as his launch angle on pitches inside the zone ranks in the bottom three percent. Meanwhile, his exit velocity on pitches inside the zone ranks in the top half of the league.
He’s a major outlier when it comes to in-zone xwOBA vs zone percentage, which seems to have a decent correlation (r = .25).
This may look like a small sample size, but plate peripherals like zone percentage stabilize fairly quickly. I think this mostly bodes as a positive for Guerrero; once he figures out how to get his swing going, the combination of his elite power and hit tools added with the fact that pitchers keep wasting pitches on him, which could all equate into some tremendous offensive value.