As title, please postpone that. It is going to hurt deeply to the current training system. The announcement is written in a very unnoticeable way (inside a paragraph which seems to be on a completely different topic). Even worse, although it is only going to be tested in scrimmages, it is going to have an immediate effect on unaware managers, because injuries in scrimmage are real.
I know that there might be a couple of threads discussing suggestions on the training mechanisms, as well as on stamina.
However, the change which gives 100% training on 18- and 19-year-old players playing 45 minutes or more will be marginal. First, I don't know why there is a reason that the same rule should not be applied to older players. Second, without having players playing full games, it is almost as hard to control 3 or 6 trainees to play 45+ minutes, compared to 48+ minutes. The main reason for this is that game outcomes are sometimes hard to predict. When a game is close, starters/benches might share the game time like 36/12. On the other hand, when the game gets into garbage time, sometimes starters/benches might share the game time like 24/24. It is usually hard to sum it to 45 consistently.
Therefore, no matter the rule is 45+ or 48+, it is still quite inevitable that managers have to put their trainees to play full games. And thanks to the change of the game, they are going to get injured more easily.
In the past, when we are doing single position training, it is not so hard to give 3 trainees full training most of the time. However, if the game engine penalizes this by increasing the injury possibility, we might be forced to train only 2 trainees at a time. This is a 33% debuff.
To be honest, I am a bit disappointed that under so many discussions, the game engine is not moving toward a better way. The rules get complicated, but all we got were inconveniences.