Neil Harris admitted Millwall’s first-half display was “unacceptable” as his side crashed to defeat at home to Preston North End.
The Lilywhites scored three goals in the opening 27 minutes in the way to a 3-1 win at The Den.
Andrew Hughes, Tom Clarke and Sean Maguire were on target for Alex Neil’s side as they made it four straight away victories in the Championship.
Ben Thompson scored with his first touch for Millwall midway through an improved second half display from the home side.
“The damage was done in the first half, which wasn't acceptable,” said Harris, whose side had won 1-0 at Derby last time out on Wednesday.
“We conceded two goals from set plays which we got all wrong.
"It wasn't just the goals, it was the lacklustre first half hour which I don't expect from my players at The Den.
"Alex Neil's pre-match teamtalk would have been about getting after us and putting us under pressure, whereas ours was about starting well and showing that we've recovered well from Wednesday.
“They got the ball in behind us quickly and with quality, and we didn't. We were a yard off it all over the pitch.
“What I will give credit to my players for is the second half display, yes, we were very attack-minded and left ourselves open on the counter-attack, but we should have got back into the game with the number of entries we managed to get into the penalty area.”