Offenses just don't work due to luck. They work because various coordinated pieces work as a unit to get a specific objective done. If you lose a RB or a WR there are backups to put in. they more than likely will not be as good as the original player but they know what their job is from play to play. When you have no backup for a position and have to sign one the week of a game you are not in a good position. It is not like they can reinvent the offense in 7 days. They can only take the plays they have and make small alterations.