This is how the numbers of gain from the games breaks down in relation to lvl 4 drills:

6 stars players:
Condition lost in games: 354
Ability gained in games: 81
TEF in practice lvl 4 drills (see below): 0.21
Ability gained if same condition loss has been used to practice (354x0.21): 83

7 stars players:
Condition lost in games: 703
Ability gained in games: 100
TEF in practice lvl 4 drills (see below): 0.14
Ability gained if same condition loss has been used to practice (703x0.14): 98

So the gain over a bit of a sample size is basically identical.

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So if you have lvl 1 drills, play your players as much as possible. If you have lvl 4 drills, from a gain point of view it does not matter if you play or train them.