In goalunited, it is not possible to manually substitute players during matches. The complete formation has to be saved at least one hour before the start of the match.
The only way to influence the match after this is to play half-time direction cards. Those can be played up to 50 minutes after the match has begun to react to an unsatisfactory first half.
Substitutions in matches are performed automatically only when there is a serious injury.
In case of an injury sometimes game may choose a better suited player for that position even if the player is not on same position like the injured player. For example; Winger player enters the match instead of injured midfielder.