Yeah, if one of your other players don't feature in this round.
The order they are substituted is set by yourself. But if you have say 3 backs on the pitch and one of them isn't featuring, then if you have the substitution order 1)Mid, 2)Back, 3)Back, then you'll get the 2:nd sub thrown in no matter what. You always have to play at least 3 backs.
After 11:30 GMT Saturday (normally) the week is closed and you can't make anymore changes to the squad.