No one answered this so I will.
2017 and 2018: Kopech starts off both years in the minors. He sets up a pattern, he struggles with his control early in the year then his control and performance improve during the year. He looks like a guy with a big arm but who needs work to find his control. In 2018, there is a game early in the year where he walks 5 in a short outing and it shows in his stats. Then he goes the month of July and barely walks anyone, he has clearly found a groove, and gets called up.
He pitches 4 games and leaves with TJS. He is out through the end of 2019. Some people have disputed whether he should have been called up at the time, it was controversial, I personally had no problem with it as I felt he had done what he needed to do in Charlotte and it was time to give him some big league innings before the season ended.
In 2020, for various personal reasons, he opts out of the shortened COVID season. While this isn't the team's fault at all, this does mean that he hasn't thrown a pitch in anger in nearly 3 years by the time he comes back in Spring Training of 2021.
So you have a guy who has a live arm, hasn't pitched competitively in 3 years, and had a couple cycles of of "Needing to build up innings so that his control would improve". This seems like a guy who should absolutely be sent to the minors to start 2021, or at least given a method to be able to start for a part of a season, he needs a chance to throw as many innings as he can. Personally I would have put him in the minors and said so repeatedly at the time, they should have been trying to push him up to 100+ competitive innings to strengthen his arm. They put him in the bullpen to start the year. Ok, fine, they moved him to starting in May when someone was hurt, and then he hurt his leg and went on the IL.
When he came back, they brought him back without any minor league rehab stint, brought him back right before the all star break so he had extra time just sitting around, and then buried him at the back of the bullpen for 2 months. He had a grand total of 2 outings that reached 2 innings in all of July and August combined. He didn't throw 3 innings in an outing again until September 29. Don't get me started on how he was used in the playoffs as that was LaRussa stupid too.
He threw a total of just over 70 innings the whole year. At the time, I guarantee you I was saying "Fine, this might work for this year, but you're taking the risk that the guy will not be able to find the kind of control he was developing in the long seasons in 2017 and 2018, and you're risking whether he can strengthen his arm enough to be a starter with this short of a season." What are his problems now? Control, use of his secondary stuff, and a few additional injuries.
Now that's not necessarily the only problem he had, 3 years off is going to be tough to come back from for anyone. He may well have other personal issues, I don't know anything about his current work ethic, and Ethan Katz may well have no good solutions for him. But in terms of mishandling him, for both Kopech and Crochet they put them in the 2021 bullpen to "Win now" and one of the risks of doing so was "This might help you right now and leave you with both of these guys struggling with injury and control if you try to move them to starters." That is precisely what happened, and I continue to believe that this is no coincidence, the White Sox gambled these guys' future as starters was unimportant compared to their 2021 success, and wound up with guys who can't handle themselves as starters.