Christopher, a High School & College referee, asks:
The keeper wearing jersey #31 was injured and replaced with a substitute keeper wearing jersey #1. This keeper then got a red card and had to leave the field. A player on the field became a keeper but wearing the same jersey #1 worn by the keeper who was red carded. Is this permitted?
This is less a matter of Law than a matter of procedure governed by the local rules of competition. Another way of putting it is that we have no particular answer to it because it is not strictly a Law question – the Law per se has nothing to say about jersey numbers, only the fact that the keeper’s jersey has to be clearly distinguishable from the jerseys of everyone else and that, in turn, is solely a matter of color (and secondarily, of design).
The purpose of jersey numbering has little to do with distinguishing field players from goalkeepers and much more to do with (a) maintaining a “clean” team roster and (b) enabling the referee to record jersey numbers rather than actual player names if/when there is ever a need to record behavior generally and cards specifically for the match report at the end of the match. For example, the local rules of competition might well have a requirement that each player and substitute have a unique number on the jersey which matches the a number identifying the player in the roster given to the referee before the start of the game. It also depends on the level of competition because, at lower levels, teams generally may not have sufficient funds to have uniquely numbered “backup” jerseys in cases like goalkeeper replacement and/or replacement of a damaged and/or bloody jersey.
By the way, note that, as part of the point made earlier that the Law has nothing to say regarding jersey numbers, there is certainly nothing requiring goalkeepers to wear any particular number (e.g., 1) or that a replacement player has to wear a jersey with the same number as worn by the player who was replaced. At most, local rules might require that any jersey numbers noted on a team’s roster might need to be adjusted as/when there is a change.