Gary Rowett admitted he had mixed emotions after his Millwall side won at Pride Park on the day Derby saluted former manager Jim Smith, who died this week.
Rowett, along with a number of ex-Derby players, took part in the pre-match tributes to Smith, who took the club into the Premiership in 1996.
But Derby's chances of marking Smith's passing with a victory were dented when Millwall deservedly went ahead in the 25th minute through Tom Bradshaw's looping header.
Derby were booed off at half-time and they improved in the second half but were unable to conjure a draw or win.
Rowett took Derby to the play-off semi-finals two seasons ago and he said: "It's no more pleasing to win here than anywhere else, probably less pleasing today in some ways because of the tribute to Jim before the game.
"It's a difficult one because if Derby win it's a great day for Jim and Jim's family and I wouldn't have been displeased but at the same time my job is to come here and try and take the three points and I'm really pleased we did.
"We looked at our away form and trying to turn it round which I felt was key and we looked at the four games we had in Swansea away, Birmingham, Bristol City and Derby we knew they would all be very difficult so to win three of them and to draw one is a massive testament to the players.
"It wasn't one of our better performances but to come here and limit them to one deflected shot on target is a massive effort from the players."
Derby manager Phillip Cocu said his team had to show "more courage" with the ball.
"It's not only the start, I see the first half and I observe and see one thing, we don't have enough courage to play," the Rams boss said. "In the second half, we do what we want to do and you see a completely different team.
"We had a few opportunities, not a lot because it was difficult with the lack of space, but the struggle we have all season is finding the net and we have to find a way to score goals.
"After we concede a goal we think now we have to play because we have to come back in the game but if it's 0-0 we have to do the same. When we have more possession, the type of players we have we need to play the game of football like we played most of the second half.
"That's why I say we need courage to play because in the first half you see the result if you don't show the courage."
Cocu has Wayne Rooney available in January but he warned: "We can't think we get Wayne in January and everything will be fine, I don't think that's enough. We need more."
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