A lot of what happens on the pitch is pre-planned: triggers for runs or when the press happens, patterns of movement and passing, and so on and so forth. Players don't actually make a ton of creative decisions on those on the fly all the time; most of what happens are variants of patterns practiced in training until they became automatic - at which point the in-game creativity is more about being able to recognize what pattern fits a situation and improvising around it as required.
If that sounds like coaches are drilling things into players through a lot of repetition, then you got it exactly right. And that's exactly what you can't do during the season if you have midweek games. Teams generally have only one on-field training session per day during the regular season (sometimes two during pre-season, but not even always), and the day after the game is for recovery. I think the game before the day is match-specific prep (learning to anticipate things that are specific to the upcoming opponent, and specific routines that will work will against them), plus there are rest days - meaning that you often have only one day between matches to work on the routines.
That's really very little and means that drilling in routines during the season is very slow at top clubs. (It's better at lower levels, since they don't have European football and will often be eliminated from domestic cups earlier.) That's why pre-season is so massively important for a new coach. As it happens, I actually saw a reference on the forum today to a BBC article that discusses this:
Link:
https://www.bbc.com/sport/football/articles/cgm9evp4177o
For my purposes, the beginning is really the key part, when Amorim mentions (well, implies) that he has had only four sessions where they could really work on drilling in his system.
(To get back to my point from a few posts ago, the subsequent bit is also interesting, cause you can see that Amorim himself also thinks a summer transition would have been preferable; but it was apparently now or never for United, so he decided to go with it. Which in turn means that United management and Amorim are both very aware of the limitations of his current situation and how difficult that makes the transition to Amorim's style - and will likely back him through that even if results continue to be poor, as long as relegation isn't a real risk.)
But to go back to the topic of this post: so this shortness of training moments is a real thing. It's not a whiny coach looking for excuses, it's really the difficult reality of bringing significant change to a club mid-season.