Kevin Nugent branded the four goals conceded in Monday afternoon's Professional Development League defeat at Watford as "really, really poor" - and challenged his squad to "stand up and be counted" after suffering a fifth game without a victory.
The Hornets were 4-2 winners in a match which saw Jayden Davis and Abdul Abdulmalik find the net for Millwall's Under 23s, but the hosts were more ruthless in front of goal, including a seven-minute period across both halves which saw three goals scored.
"The four goals we conceded were laughable. Really, really poor," Nugent told millwallfc.co.uk. "I don't think the game was a 4-2 defeat, it could've ended up 5-5. Their goalkeeper made some saves and we had some good attacking play during the game.
"However, you can't concede four goals, especially in that manner, and expect to get anything out of the game. The goal just before half-time was a difficult one to take, but all of them were preventable."
Nugent revealed his post-match message to his players as the Under 23s attempt to dig themselves out of a rut which has seen them winless in five.
"There are going to be times when you are lacking in confidence or self-belief, and you've got to find a way to get out of it and play through it. They've got to find their feet," he said.
"The team is consistently changing, which doesn't help, but we need people to stand up and be counted - hopefully they will do that against Swansea in our next game."