A plug is a hockey player than sits in the press box most nights. They only make their way into the lineup when several injuries occur, leaving the coach with no other choices. The player is “plugged” into the lineup.
Richard: “Damn, dude, did you hear that Melvin and Marvin are both hurt and can’t play tonight?
Murphy: “Yeah I saw that shit. Now Philly and Dill are lacing them up tonight. Those guys are such plugs. We’re fucked!”