I think you should be able to set up a default week (three games) instead of just a default roster so if you could not sign in for a week you would be okay.
The game is designed so that you have to log in regularly to have the most success. The default line-up exists so that you don't lose an odd game here and there when you're unable to log in.
Setting a line-up for a whole week is thus out of the question, really. First, you can do it even now: just go in your schedule and set up all three games. Second, it's not going to really help you, since lineups might need to be reevaluated: injuries, foul troubles and blowouts can all cause you to alter minutes for your players.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."