Warnock says Murphy's exclusion from his squad for their crucial clash at Fulham was due to the winger's poor showings in training this week.
"He's been poor in training. No excuse," a frank Warnock explained after the game.
"I could have said he had something creaking in his back or something, couldn't I? But he's been poor in training.
"I hope it gives him a kick up the backside and he's better in training next week."