Kevin Nugent labelled his Under 23s side's performance at Cardiff City as "unrecognisable" as The Lions slipped to a 2-0 loss.
Chanka Zimba and Isaak Davies scored at the Cardiff International Sports Stadium in the Professional Development League, and in speaking to millwallfc.co.uk, the manager expressed his frustration.
"It was a disappointing performance, especially after last week's one, which was so good [a 4-0 victory over Barnsley]," he said.
"We were unrecognisable. We lacked passion and desire and Cardiff physically outbattled us.
"We were better in the second-half, but by then it was too little, too late. That's something the lads will have to learn from. They took the plaudits after last week's win, but now they will have to take the criticism. They must learn from both sides of the coin."