American sports coaches get around that with exceptionally cryptic signals and numbered plays, but football isn't nearly so tactical. I get the impression that most of the tactics a manager might call out mid-game are simple fire-fighting, e.g. instructing a winger to sit on a marauding wing back.
The opposing coach often has to wait to see if the change is effective before deciding whether to change anything.
More complicated changes are often carried onto the field by a substitute who's had them whispered into his ear.