Soccer confusion

I'm still having trouble digesting this.
Playing with just seven players in a sport where you normally have 11 on the field, Calvary went out and built a 4-2 lead over an opponent with under five minutes remaining when it suffered another injury.
While it was OK to start the game with seven players, apparently you can't play soccer with six players. As a result, Calvary was forced to forfeit a game it clearly was going to win.
I suppose Calvary could have left the player on the field and out of harm's way to finish the game. Instead, the team did the right thing for the player, and the reward team is it's forced to forfeit the game.