yes, but there are 2 league games each week, and either a Cup game or scrimmage, for a total of 3 games each week. If a player plays alot of minutes then his chances of injury increase and "Practice this week" goes down, which means they will not play up to their potential.
and allowing for injuries, you probably need to keep most of your original 15, and can parse down a few. I now have 20 on my roster after getting 3 in the draft, and I have sold off one, and am going to fire 1-2 in the coming weeks.