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Slugging is a fairly simple metric by baseball standards but once Statcast gets into the mix, it's a bit more complicated. According to MLB.com, "Expected Slugging Percentage (xSLG) is formulated using exit velocity, launch angle and, on certain types of batted balls, Sprint Speed." Base hits are still weighted with home runs being four times as valuable as singles, but the expected statistic focuses on the process rather than the result to determine what should have happened.
For that reason, most players will see a discrepancy between their actual slugging and expected slugging rates. Often, although not always, this means regression is coming for the biggest overachievers and underachievers. This can be extraordinarily helpful for fantasy baseball managers looking to gain an edge by buying low or selling high on players at the right time. Here are three such players in each category.
All Statcast metrics are current as of June 14, 2021 and data/graphs are taken from BaseballSavant.mlb.com.
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xSLG Underachievers
Alex Kirilloff, Minnesota Twins
-.176 SLG-xSLG
We've covered Kirilloff before, but he continues to be a Statcast anomaly based on an unpredictable start to his rookie campaign. His four homers in three games back on April 30-May 2 led to a frenzied excitement only to have him land on the IL two days later. Since his return, he has gone 19 straight starts without a home run. He only has two extra-base hits in the month of June. Is the wrist injury sapping him of his power?
It doesn't appear to be the case, as Kirilloff is maintaining a strong 31.8% line drive rate and 50% hard-hit rate. A launch angle adjustment may be in order if more balls are to leave the yard. Line drives are great for singles and the occasional double but we're hoping for long flies out of this power prospect. The tools are there and his quality of contact, as well as a 5.5 xHR above his four actual HR is promising enough that we can hold out hope for more counting stats eventually.
Michael Conforto, New York Mets
-.135 SLG-xSLG
While he doesn't come close to earning the award for most disappointing Mets hitter, Conforto comes in a close second. A strained hammy has kept him on the shelf for nearly a month now, but his prior performance can't be overlooked. Just two homers and 13 RBI in 33 games (113 AB) is not what we were hoping for. Neither is a .230/.356/.336 slash line after he went .322/.412/.515 the year before. After ranking among the league leaders in OPS with a .927 mark in 2020, that figure has plummeted to a .692 OPS.
Conforto's underlying Statcast metrics show little movement in terms of exit velocity and he's improved his plate discipline. His ground ball rate has gotten to a career-high 45.3% but that is simply league average even if it's high for him. This mainly is a case of the pendulum swinging the other direction. Conforto was a big overachiever last year in xStats and is now experiencing the other side. Once he gets right and back into a groove, we could see a second-half surge.
Alec Bohm, Philadelphia Phillies
-.115 SLG-xSLG
Those who drafted Bohm to be their starting third baseman after an impressive rookie showing will be glad to know that his expected stats indicate he could see significant improvement soon. Bohm slugged .481 across 160 at-bats in 2020 and his .474 xSLG shows that he earned it. His numbers have taken a huge dip across the board, especially his .309 SLG. However, his xSLG is far higher at .424.
It's not as if he was an extra-base machine last year, going deep four times and accounting for 11 doubles. It was expected that he would transfer his strong hit tool and raw power into more game power in his second season. Instead, he has the same amount of home runs and seven doubles in 73 more at-bats. His hard-hit rate, exit velocity, and sweet spot rate are all better than 2020. His plate discipline is worse though, going from an 11.1% K-BB% to 20.8%. Breaking balls have been to blame, as he's seen a .273 drop in slugging on breakers alone.
The good news is that he's getting better at hitting offspeed stuff. If he can solve his issue on breaking pitches, things will start to fall into place.
xSLG Overachievers
Joey Wendle, Tampa Bay Rays
+.127 SLG-xSLG
Mr. Wendle is the biggest overachiever among qualified batters on the xSLG leaderboard. It's not as if his .495 SLG mark is astronomical, but his profile says he doesn't belong in a Major League lineup much less a fantasy one.
Wendle has a career .417 SLG mark including this year, so his .495 clip looks out of place. His 34.2% hard-hit rate is nearly identical to his 34.6% career rate and nothing else suggests he should be doing better than usual. Wendle is a solid fill-in for an MI spot or a utility player in case of need, but the sudden threat to reach double-digit homers for the first time in his career shouldn't be taken too seriously.
Yuli Gurriel, Houston Astros
+.107 SLG-xSLG
One of the biggest turnarounds from last season, Gurriel has transformed a .232/.274/.384 slash line into a .323/.392/.536 slash line in 2021. This isn't the best he's ever slugged, having reached a .541 SLG in 2019. That year, he went deep 31 times and drove in 104 runs. The 37-year-old is on pace to repeat those numbers, but can it hold up all season long?
If history repeats itself, he most certainly can. Gurriel's xSLG two years ago was .407 and it's higher now at .429. Despite the fact he is far exceeding his expected stats, he's done it before and has shown consistency in his xSLG profile.
Even in his down season, Gurriel kept a similar level of contact quality. That includes an exit velocity that hasn't waned whatsoever with age.
Gurriel is a veteran hitter who has stayed in shape and has a very consistent approach. His impeccably low strikeout rates allow him to avoid prolonged slumps and his lineup protection is one of the best in the game. There is no reason to doubt that he will enjoy one of the better seasons of his long career.
Cedric Mullins, Baltimore Orioles
+.100 SLG-xSLG
Sorry to be the burster of bubbles, but Mullins' unexpected power surge might be near its end. This is a player who, up until this season, had averaged 53.4 AB/HR in the majors and is now averaging 27 AB/HR. We can't point to a lucky HR/FB rate, since 12.2% is pretty reasonable (even lower at 8.9% according to Baseball-Reference.com). His homers haven't been cheap either, with 8.6 xHR and only four counting as "Doubters" according to Statcast. A power surge for a 26-year-old in Baltimore finally getting playing time is pretty reasonable too.
Going back to Statcast, as this series is intended to do, his xSLG is middling in the 48th percentile. There aren't a steady stream of barrels and nothing screams sustainable power. He is pulling the ball much more this year and maximizing launch angle to get a hold of the fastball better. He was already a good fastball hitter, now he can drive it out of the park repeatedly. A 20-HR season could be in the works so it's not as if I'm going to declare the sky is falling on Mullins, but his main calling card remains speed. He won't be the next Mookie Betts. Think more Victor Robles a la 2019.
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