Have a look at the defense of my last league game. Yes, it sucked. I usually have a pretty good defense. I had actually saved the orders with the usual defensive players. Only 30 minutes before the game, hence too late to change, I noticed that a linebacker had been injured. Does the game automatically change my whole defensive line-up because I had an illegal formation due to the injury of a player? What happens when a player that is part of the line-up is injured? Is he replaced or is the whole formation messed up?

the whole defence will have been altered as if you had not set any formation... because the player was injured, the formation becomes illegal, in that situation the game treats all illegal formations the same, and plays a default arrangement with random players...

Yup that sucks injuries are tuff when happen friday and play is saturday. Just have to log in friday night or saturday morn.