Kevin Nugent has urged his players to "keep improving" following Tuesday's defeat to Sheffield United in the Professional Development League.
Sean O'Brien's second-half penalty-kick was sandwiched by goals from Zak Brunt (pen) and Kyron Gordon as The Blades claimed the Professional Development League (North) title at Calmont Road.
Speaking to millwallfc.co.uk, the boss told of how the first 10 minutes of the match cost his players a victory.
"My last words before going out to play were that no matter what level you play a Sheffield United team at, they are going to put you on the back foot straight away and force you into mistakes," he said.
"Unfortunately, the first 10 minutes have cost us because we weren't fully up for the game. We made an error just outside our box which led to them getting the penalty - we should've just cleared our lines.
"It's a learning curve, though. In the latter part of the first-half and the first part of the second-half, we certainly deserved something out of the game. However, I'm quite tired of saying that now."
Tyrese Briscoe was introduced at half-time and gave The Lions added attacking punch, with the striker winning the penalty converted by O'Brien.
Nugent explained how his message to the players at the break was to implement the "basics of football" - something Briscoe did following the restart.
"I said to the players at half-time to never underestimate the basics of football - running around, putting yourself out and physicality. They give you a platform to play from. If you don't have those then you're going to struggle.
"The goal was against the run of play, but once again we've had enough chances to pick something up and we haven't taken the points. There hasn't been too much wrong, but we're getting punished for our mistakes. We've got to keep improving."