That solution is pretty good. If you stuff simply raw recruits in, then it negates the effect of retraining etc.

BTW, the numbers system is fairly broken. Since morale penalties apply to %casualties taken counting from the start of the battle, then it would be better to have equal numbers of smaller units. This is also next to the maneuver advantages.

I.e. compared to 1 unit of 20 men, 20 units of 1 man each is strictly better (assuming exact same troops) as you'd never see any units rout in the 1 man/unit case.