I did spend some time going through the stats on his huge 2021 dropoff. McNeil has an excellent K-rate, but a generally low exit velocity. He doesn't hit the ball hard, but he makes contact. Compared to 2020 his walk rate did drop off somewhat, but his K-rate didn't spike and there was basically no real drop in his exit velocity compared to 2019, and his exit velocity was higher in 2021 than in 2020.
What I do see is that there's a big change in the way he's hitting in 2021. There's a big dropoff in him pulling the ball and a big increase in him going the other way. There's a drop in hard contact and an increase in medium and soft contact. There was a career high in ground ball rate.
His BABIP went from being .340 in his first 3 seasons to .280 in 2021 - was that bad luck, or was that the poor quality of his contact? It's a little hard to believe his BABIP stayed above .330 all 3 of his first 3 years and 1000 PAs by luck. Perhaps a little bit unlucky in 2021, but the dropoff in BABIP fits with the dropoff in his contact quality.
He also went from a 15% HR/FB rate in 2019 to a 6.5% rate in 2021 - but its hard to see a high HR rate as sustainable with his generally fairly weak contact.
One thing that does stand out, going along with the drop in pulling the ball, increase in groundball rate, and drop in HR rate, is that his launch angle has noticeably dropped.
Now here's one final problem - since Statcast started recording, the White Sox have had as low of a launch angle as any team in the league, bottom 5 in the league 6 times and bottom 10 in the other season. So, you've got a guy struggling badly in 2021 when he's going the other way, hitting the ball on the ground, and not elevating like he did beforehand. For whatever reason, the White Sox are really bad at launch angle, so if a low launch angle and not pulling the ball are big parts of why he was terrible in 2021, do we think the White Sox can fix that? Or is this a guy who is a particularly bad fit for the White Sox since his success was associated with a higher launch angle and that's a weak spot for the White Sox?