Wayne Rooney offered 5,000 travelling supporters a reminder of former glories but, thanks in part to two goals from new striker Odion Ighalo, Ole Gunnar Solskjaer’s current Manchester United side made another potentially significant step forward.
A rejuvenated Luke Shaw was also on the mark as the 12-time FA Cup winners advanced, and a defence that was heavily criticised earlier in the campaign recorded a seventh clean sheet in their last nine games.
And two goals in eight minutes late in the first half as good as killed off the tie and had Solskjaer planning that quarter-final trip to Norwich.
In keeping with United’s strong, nine-game unbeaten run, it was the identity of those goal scorers who summed up the strides being taken by the Norwegian’s squad as they approach the business end of the campaign.
After 33 minutes, here was Luke Shaw cropping up to open the scoring and ease the pressure on his side.
And before the interval, Ighalo marked his second start for his team with his second goal. Again, Solskjaer had been held to ridicule in some quarters when he moved to sign the former Watford front man on loan from China in January but the Nigerian has already made his mark.
But this fifth round tie , of course, was not about current United players, it was about a former one as Rooney took the plaudits from both sets of supporters as he faced the Reds for the seventh time.
The 34-year-old is still waiting for his first career goal against them although it was not for his lack of trying here and his 18th minute free-kick, awarded after Shaw was booked for a rash challenge on Louis Sibley, had Sergio Romero diving smartly to his right to turn around his post.
But that was as close as the Championship side came to opening the scoring as United began in cautious, but solid, fashion, probed and felt out their opposition and then slowly built momentum towards that opening goal.
It came from a move started by Shaw on the left and, after his cross found the hapless Lingard whose poor shot was blocked, Bruno Fernandes’s strike fell for Shaw who drove the ball in, on the bounce, from 15 yards.
The shot actually took a slight deflection off the back of Lingard although all indications were that the goal would be credited to the left-back, whose only other in his career came on the opening day of last season against Leicester.
United, and Shaw, grew in stature and, after Derby defender George Evans escaped a handball appeal following a Fernandes shot, United doubled their lead.
Shaw, again, was involved with an extremely well-aimed pass which found Ighalo in a sliver of space and the striker squeezed between two markers before burying his shot past Kelle Roos.
United, last cup winners under Mourinho four years ago, survived a penalty appeal of their own for handball after Martyn Waghorn’s shot struck Victor Lindelof although the loss of Tom Lawrence to injury did not help.
Still, the Rams opened the second half with a long shot from Max Bird which Romero gathered well and a cross from Jayden Bogle which Waghorn glanced just wide with a committed header.
It was a bright start to the period from a young Derby side — younger than some Rams fans would have hoped to see given the fact their league season looks doomed to end in mid-table anonymity.
Derby pressing forward merely led to the opening of space for United to hit them on the counter-attack; half chances coming and going for Shaw, Luke McTominay and Juan Mata before Ighalo helped himself to his second goal of the evening.
The striker started the move before his right-foot shot was blocked and he responded with a first-time left-foot swing at the rebound which ended with the ball flying into the roof of the Derby goal.
There was time for a late, brilliant Rooney free-kick that was superbly kept out by Romero once more, a moment that summed up a frustrating evening for United’s record goalscorer.
DERBY (4-2-3-1): Roos 6; Bogle 5, Evans 5, Forsyth 5, Lowe 5; Bird 6, Rooney 7; Knight 7, Sibley 7 (Shinnie 80), Lawrence 6 (Whittaker 44, 5); Waghorn 7 Marriott 67, 5).
MANCHESTER UNITED (4-2-3-1): Romero 7; Dalot 6, Lindelof 6, Bailly 6, Shaw 9 (Williams 80); McTominay 7, Fred 7 (Martial 73, 6); Lingard 6, Fernandes 7 (Pereira 67, 6), Mata 7; Ighalo 8.
Referee: Craig Pawson 7.