Each national association is given authority to set the minimum number of players in a match. The United States Soccer Federation has established this number at seven, one of whom must be identified as a goalkeeper. (Law 3 -- IFAB Decision 1).
Accordingly, the referee must abandon a match if a team either cannot or will not field at least seven players. This could occur as a result of a team having any combination of five or more players who:
- had been sent off the field for misconduct,
- failed to appear for the match despite being listed on the roster,
- were unable to play due to injury,
- had departed from the area of the field, or
- were unwilling to enter the field despite being present.
However, a match may continue if a team drops below seven players on the field as a result of a player requesting and receiving permission from the referee to leave the field temporarily (for treatment of an injury, for example) or a player being instructed to leave the field to correct equipment or for bleeding from a wound. In such cases, the referee should be satisfied that the team will be able to field the minimum number within a reasonable period of time as a result of the return of such players.
If a match has been officially started but must be abandoned due to a team not being able to field the minimum number of players (for whatever combination of reasons), the referee must provide a full report to the competition authority which includes information regarding the circumstances under which this occurred, what efforts were made to determine if qualified players or substitutes were available, and the score existing at the time the match was terminated. Before declaring the match terminated, the referee must determine if the rules of the competition provide for any alternative action.
In the absence of any rules from the competition authority requiring a different result, the team which was able to field the minimum number of players is declared the winner of the match, regardless of the score at the time the match had to be terminated.