Kevin Nugent bemoaned the "small details" that are hindering his side's ability to pick up results in the Professional Development League.
The Lions came from behind against Birmingham City on Tuesday afternoon, with Arthur Penney and Jayden Davis finding the net, but were beaten 3-2 as Ryan Burke scored the winning goal.
Speaking to millwallfc.co.uk, the Under 23s boss explained the need for his team to tighten up at the back and become a team who are "really hard to play against" once again.
"The lads are really disappointed because they feel as though they've done enough to get something out of the game, even though we were 2-0 down at half-time from two very good strikes," he said.
"However, we've now gone from being a team who in the first half of this season, and the last couple of years, have kept clean sheets and been really hard to play against, to a team who are conceding goals on a regular basis.
"That's not a coincidence - some lads need to improve on small details in games. Equally, though, they showed good character to come back from behind today and they had enough chances to win the game as well."
Two players absent from Tuesday's encounter was Junior Tiensia and Sam Skeffington, who have began loan spells at Sutton United and Barnet respectively.
"The two lads have gone out on loan and will hopefully get some minutes at a good level. The main aim in Under 23s football is to get the lads out there playing at first-team level."