When placing a player on IR, you have to do it for TOMORROW'S lineup. You are forced to have him on your bench for today's lineup. Once you de-activate the IR player on the next day's lineup, you are free to add a player who will be available tomorrow (unless he has a waiver period). You have until about 3am(EST) to pick someone up for the next day. Any player picked after 3am(EST) will not be available until the next day.