Team sheet

It isn't only captains who need to know what players are in their team - umpires need this information too! Otherwise all kinds of chicanery could take place - with good batsmen being replaced by demon bowlers once the side starts fielding, and so on. A team sheet, bearing the names of the nominated players, must be handed to the umpires prior to the toss being made.
If one or more nominated players have not yet arrived, a captain can ask permission to use a substitute or substitutes until they do. The umpires will judge whether the reason is satisfactory.
The team, once nominated, cannot be permanently changed unless the captain of the other side gives his consent to a replacement player. If he does consent that player then takes up a full role in his side, just as if he had been a member of the original nominated team.