Neil Harris stated that he left his players in "no uncertain terms" regarding The Lions' second-half performance in their 2-1 New Year's Day defeat at Norwich City.
Steve Morison nudged Millwall in front with an audacious goal shortly before half-time, but strikes from Tom Trybull and James Maddison earned The Canaries all three points after the restart.
The manager expressed his disappointment in the way both goals were conceded, as well as branding the 45 minutes "a disgrace."
"I'm raging with my players," he said. "To weather the storm after the first 20 minutes and to deny a really good footballing team to so few opportunities and then get our noses in front - the job is half done.
"To then concede two goals in the manner we did - the second-half performance was a disgrace from my players. We didn't get near enough to the ball and the concession of the first goal is abysmal by us. To give away a free header in that position is not acceptable.
"I've left my players in no uncertain terms regarding my thoughts on that second-half. We need to be better than that.
"Concentration, focus, leadership - all the things that are fundamental when you go away from home all went wrong. At this level, you cannot give good players and good teams a leg up. The players have been brilliant for me over the course of the season and especially in the last few games, but you cannot let standards drop. That's what they did in the second-half."
Harris then went on to explain that The Lions will look to add bodies to the squad in the January transfer window, as well as praising Morison's excellent first-half strike.
"It won't be hard to lift my players for the next game, because I've left them in no uncertain terms about my feelings. People need to be on their toes in January - every window we have signed better players and that has to continue now.
"Steve's goal was a brilliant finish and worthy of winning the game. I'm disappointed, because that should be talked about tonight as the winner in a tight game. In the first-half he was a handful and fully deserved his goal."