DERBY head coach Steve McClaren admitted his side had no answer to Millwall's rearguard action at the iPro Stadium.
Steve Morison's 61st-minute header handed the visitors a priceless away win on Saturday.
McClaren claimed the visitors had "done a job" on the Rams and accused the Lions of time wasting – saying there should have been 15 minutes of added time.
But the former England manager conceded that Derby were toothless in attack and had not deserved anything from the game.
McClaren said: "If we'd been playing for two more days we wouldn't have scored. We had no quality in the final third and chances and half-chances that we normally take we didn't.
"They came to do a job and they frustrated us – and frustrated the crowd – and in the end, took the three points away.
"There's not been many times when I have said I've been disappointed with the performance but today is one when I can.
"I think teams are coming here, parking the bus, making it difficult and we didn't today have the quality to get the goal.
"Opponents are going to come here and frustrate us and we have to deal with that – but we didn't deal with it very well."
Asked about the high number of stoppages during the second half, McClaren said: "It was unbelievable the goalkeeper was only booked late on.
"We're not making excuses, we've done that before and opponents will do that. We have to deal with that and we didn't, but there should have been 15 minutes injury time."