There could be many different ways of doing this. Could you please provide a bit more detail, links to examples and possibly screenshots of what exactly you're trying to do?
Masking, shoot on a green screen, rotoscope... There's no easy way to take a shot from a game and turn one of the players into a silhouette.
Provide a snapshot of the footage and tell us exactly what you plan on doing. You're being far too vague...