Ive just recently came back so dont take my word for it, but I would start now, definitely before the season starts. A few of my players are 3 days off and it hurts.
Had to sacrifice my free players who had custom equip because they missed 2 games as well.
Zzz
p.s ive noticed since ive been gone, even if you make a player late they get all of the training pts they SHOULD have if you made them now, i would make them now though.