When I've had guys that were definitely never trained, their DMI at 9 GS remained constant, other than a 100 point reduction every two weeks that I would guess is due to a constant rate of stamina reduction (for guys in their mid 20s). I have data for this covering this season and most of last season.
If you are seeing increases, I can only believe it's due to training, where the player got some of the appropriate minutes.