Derby's play-off hopes were dealt a blow as Millwall grabbed a vital 1-0 win at Pride Park.
It was the Lions' second win in 18 away Sky Bet Championship matches and moves them five points above the relegation zone while Derby remain two points outside the top six.
Derby could have few complaints after a desperately flat performance despite Ashley Cole making his full debut and Millwall thoroughly deserved the victory secured by Jed Wallace's second-half strike.
Cole's inclusion was no surprise after his performance as a second-half substitute in the FA Cup defeat at Brighton but he was one of only two changes while Millwall also made two from the weekend.
Frank Lampard had been in the same team as Millwall boss Neil Harris at Brentwood School but 12 places and 20 points separated them going into the game, although the visitors began confidently with Mahlon Romeo whipping a ball across the six-yard box.
Cole showed his experience to get across and break up an attack but Millwall were content to sit in and make it hard for Derby although Duane Holmes made a strong run down the right in the 21st minute only for Martyn Waghorn to over-run the ball.
When Millwall conceded a free-kick 25 yards out, Harry Wilson placed the ball but could not get it over the wall and although Tom Huddlestone tried to switch play with some sweeping passes, Derby were being frustrated.
Huddlestone's 25-yard shot was just over in the 40th minute but neither goalkeeper had been called upon in a poor first half.
Derby had a chance five minutes into the second when Fikayo Tomori's ball put Jack Marriott in between Millwall's central defenders but he lost his balance and scuffed the shot.
There was a scare for Derby when Wallace skimmed the bar with a cross in the 55th minute but Millwall stunned the home side with a breakaway goal in the 72nd.
Shane Ferguson released Wallace who ran into the box before driving a low shot under Kelle Roos, the first shot on target in the match.
Derby reacted with a double substitution but they needed a spark on the field as Millwall continued to play with discipline and resilience.
A free-kick in the 86th minute found its way to Richard Keogh at the back post but his ball in was cleared as Millwall's defence held firm.
Jordan Archer punched away a corner in the first of four minutes of added time before he made his first real save to parry a Tomori header and that was Derby's last chance as boos rang out at the final whistle.
