Neil Harris has branded Saturday afternoon's late defeat at Norwich City as 'unacceptable' after conceding two injury-time goals to lose 4-3 at Carrow Road.
Tom Elliott gave Millwall a half-time lead, before Teemu Pukki equalised shortly after the interval. Ben Amos' penalty save denied Pukki a second before Moritz Leitner put the hosts in front.
Two quick-fire strikes from Ryan Leonard and Jed Wallace for The Lions stunned the home faithful, before Jordan Rhodes and Pukki turned the game on its head once more in Daniel Farke's side's favour.
"We're obviously very disappointed to be moments away from our first away win at such a difficult place to come, and then to get nothing out of it is extremely difficult to take," Harris says.
"I'm frustrated for the players and for the fanbase who I thought were excellent and really got behind the boys. The game had everything. We got in front in the first half, and we knew they were going to come strong in the second. To go behind and then get the two goals to take the lead again was great testament to the character, belief and quality of the players - especially an outstanding third goal.
"To throw it away like we did was a lack of professionalism by us as a group. We went to a back five to shore things up, with out-and-out defenders, some of them international players. We weren't good enough at the end, and that hurts to say. I'm not going to pull the wool over anyone's eyes, we shouldn't be conceding two goals with a back five like we did in injury time."
The manager states that the team will train hard to recover from the result, and outlines positives taken from the match.
"We've got a disappointed changing room, and we'll all feel the hurt tonight. For myself because I want to get points, for the fans who have travelled and for the dressing room who put so much into that game, we'll look at individual and collective mistakes. But we'll do what we do and dust ourselves down - we find ourselves in a very similar position to this stage last year - and we have to find a way to ensure we improve.
"We've got a two-week international break now to improve and be better, both defensively and going forward, and come back strong for two big home games after the break.
"What I will say positively is that after Brentford where we were really poor, some of our football today was excellent. We showed some real moments of quality and patience, our movement and use of the ball was better and we scored three goals away from home against a side up at the top of the League."
Harris says the game must be assessed honestly, and insists the defensive display 'will not happen again'.
"[The players] are good men, they're honest and professional, so I believe they will go away and look to improve themselves. I don't need to have stinging criticism of the players, because they know themselves. Players have to have that mentality - to concede two in injury time is unacceptable.
"I'm at the front of the ship - I take responsibility first and foremost. But they do have to look at themselves and be better than that.
"Our defensive display will not happen again, because we'll work day-in, day-out on it, and we'll make sure that when we face Bolton we'll be much better defensively."