This really is completely wrong. First off, there's the empirical evidence, you watch any high level hockey game, who's deciding who goes out on the ice? The head coach. Most NHL games have sound proof glass so you can't hear the coach but the NHL hadcoach does indeed pick the defensive pairings.
It's no different in junior hockey. I've sat behind the Kitchener bench plenty of times. You know who's deciding Murphy gets 40 minutes a night? It's Steve Spott. Not the associate coaches.
They may handle them in practice, but in high level hockey, the head coach is the only one who decides who's going out on the ice. The assistant coaches may give the orders, but it's the coaches choice. If you see an assitant coach sending out the defense, they're probably also sending out the forwards, on the head coaches whim however.
Secondly, you can tell this is false merely through logic. It's very inefficient and very counterproductive to have two coaches working against eachother. They would send people out for very different reasons, the chemistry would not be there. The defense's job is not only to shutdown the other teams forwards I hope you realize. The head coach absolutely deterimes the ice time for defensemen. This isn't football where there's a defensive and offensive coordinator. The head coach trumps all, especially with line changes.
Then why is this even a discussion? Murphy was far from the only player who had a mistake (His first game wasn't that bad, but people tend to only remember one thing), but he was one of the few players who had a dominating game, when Murphy domianted in the next two games.