Platoon Performance and Resulting Plate Appearances, a Quick Review

The 2024 season is off and running and, after uncomfortably long treks through free agency, many of the recently yet unsigned players have found homes in these last few days, or weeks. However, while Blake Snell, Michael Lorenzen, Jordan Montgomery and others are now signed, Brandon Belt is not.

Belt spoke to his enduring free agency recently, expressing confusion over his status. And a confusing situation it is! Belt, coming off a 2.2 fWAR 2023 campaign that featured a 138 wRC+ and a robust 15.1% walk rate, has not garnered the attention many expected that he would. A likely factor in his status is certainly age, as 2024 would represent Belt’s age-36 season. Another factor is probably versatility; Belt is limited to first base and/or DH duties at this point in his career and additionally, as a left-handed hitter, batted almost exclusively against right handed pitchers in 2023. More specifically, of his 404 plate appearances in 2023, just 39 came against left-handed pitching.

Roster flexibility is important and, when that fact is taken alongside Belt’s age and lack of versatility, it has seemingly led to teams deferring to other options. While free agency surely hasn’t played out how Belt would have liked, I have a suspicion that his obvious skill set, while somewhat narrow, will eventually win out in the form of a major league contract.

Belt’s situation did catalyze this post though, as it nudged platoon players into the spotlight. Taking a step back from Belt, I wished to better understand platoon splits in 2023 and how batter performance against pitchers of either handedness in turn impacted who those hitters faced.

For data, I relied on FanGraphs’ splits leaderboard which allowed me to pull all batters with at least 25 plate appearances against pitchers of either handedness and at least 300 PAs in total. In total, 290 hitters qualified using these PA thresholds in 2023. With numbers on hand, one can take a look at how platoon splits, as measured by wRC+, impacted the share of plate appearances for batters against pitchers of either handedness.

There are a couple things to note on the scatterplot above. First, note that the vertical axis (y-axis) represents the fraction of plate appearances each player had against left-handed pitchers while the horizontal (x-axis) represents each batters wRC+ against LHP minus their wRC+ against RHPs. Players who performed better against LHPs in 2023 relative to RHPs sit on the righthand side of the scatterplot while those better against RHPs relative to LHPs sit on the lefthand side.

Next note that the horizontal turquoise line above sits at 26.4%, which is the fraction of all pitches thrown in 2023 by left-handed pitchers. Therefore, if a batter saw ~26% of pitches from left-handed pitchers, they likely were not subject to much platoon split creativity.

Finally, on average, qualifying batters that were better against LHPs did in fact see a greater relative fraction of their plate appearances against LHPs; the orange linear regression line denotes that positive relationship.

With all that noted, the data is very noisy and there were plenty of exceptions to the general rule that batters are more often (relatively speaking) facing pitchers of a handedness that they are better against; not all players are like Brandon Belt, who had just 9.7% of his 2023 PAs against LHPs and correspondingly had a wRC+ against LHPs that was 82 points lower than against RHPs (he was protected against lefties for good reason, it would seem). Meanwhile, plenty of hitters who fared better against LHPs in 2023 saw fewer then ~26% of their plate appearances against lefties; similarly, the opposite is the case for many righty-advantaged batters taking a disproportionate amount of LHP-facing plate appearances. One clear example is that of Gabriel Arias, who had a wRC+ values of 119 and -11 against RHPs and LHPs, respectively; nonetheless, a considerable 34.5% of Arias’ 345 PAs in 2023 came against LHPs.

While the relationship between platoon wRC+ splits and platoon PA splits is not so strong or explicit as I might have guessed, there are some obvious reasons for that being the case. Namely, that players may not have drastic splits to begin with, are very simply their teams’ best options, or despite having splits, are still above, or below, average hitters against pitchers of either handedness.

For instance, in 2023 Corey Seager had a wRC+ 45 points higher against RHPs than LHPs… but still had a wRC+ of 138 against LHPs. Despite that platoon split, Seager is a good enough hitter against all pitchers to warrant plate appearances regardless of who is on the mound and what arm they pitch with.

Given the presence of those Corey Seager-types, it made sense to further limit the data to those players that might be referred to as “obvious platoon candidates”, defined here as those with above average production against pitchers of one handedness and below average production against pitchers of the other handedness. Including only those players provides a clearer look at how often any given “obvious platoon candidate” is getting disproportionately more PAs against pitchers of their desired platoon split than the league at large.

First, the data was limited to those whose performance was better against LHPs.

Of the 65 qualifying lefty-favoring platoon players, 39 did in fact gets >26.4% of their plate appearances against LHPs. While that means 60% of lefty-favoring hitters were matched up at above average rates against LHPs, it also means 40% of those players were not facing LHPs so often as the league altogether, despite being below average hitters against RHPs. The lowest right-most point in the scatterplot above is represented by Tony Kemp, who received just 15.3% of his PAs against LHPs despite having a 167 wRC+ against them in 2023; meanwhile, roughly 85% of his plate appearances came against RHPs, who he had just a 61 wRC+ against. From that particular perspective, it would seem as though his presence in lineups was somewhat mismanaged.

The story is not dissimilar in the case of RHP-favoring hitters.

In this case, of 68 qualifying hitters with above average production against RHP and below average production against LHP, ~62% (42) did in fact face RHPs more often than the league overall, as measured by the fraction of all pitches coming from either hand. Again though, 38% of RHP-favoring hitters then faced RHPs less often than league average.

While generally speaking hitters are facing pitchers more often based on their performance against either handedness, there are plenty of batters whose distribution of plate appearances against either pitcher handedness in 2023 didn’t necessarily align with their performance. Among all hitters that were above average against pitchers of one handedness and below average against the other, roughly 40% of that group was getting more plate appearances against their weak platoon opponents than would be ideal relative to the overall breakdown of pitcher handedness.

And with that we are back where we started on Brandon Belt and roster flexibility. MLB has clear rules for limiting the mix-and-match inclinations of any given manager – for both batters and pitchers – and for good reason. In baseball, you never know who will come up in the biggest spot or might be the last, best option as a bat coming off the bench. For batters with clear platoon splits in 2023, managers managed to get just ~60% of those players into favorable matchups more often than chance alone would seem to have. In a game where flexibility and roster construction offer decent levers to circumnavigate rules, those players who aren’t so flexible might find themselves on the losing end of things.