Yeah that's true. But I assume it's exponential. At least it should be, almost all formulas in Hattrick are exponential, just an example of course.
I thought someone said before that whne you got under 48 minutes it had a huge negative impact, so that should mean exponential, but the rules says nothing about it;
If they play more than 48 minutes, this is not a problem. If players play under 48 minutes, they will still improve, but less quickly than they would if they got more playing time.