Match ends, Derby County 1, Manchester United 3.
Manchester United relieved the pressure on manager Louis van Gaal by beating Derby in the FA Cup fourth round.
Van Gaal dismissed claims he offered to resign after losing to Southampton on Saturday, and his team were more positive against the Championship side.
Wayne Rooney curled in from 20 yards to put United ahead before George Thorne cut through their defence to equalise.
Daley Blind then finished low into the net from Jesse Lingard's cross before Juan Mata sealed victory late on.
Derby, who have not won a league game in 2016 and are fifth in the Championship, caused problems towards the end of the first half when Thorne latched on to Chris Martin's delightful through ball.
But that aside, it was a confidence-boosting win for Van Gaal's team who, after a winless December, have lost once in January and registered a two-goal victory for the first time in 15 matches.
Martial helps United rediscover their swagger
It has been a troubling week for Van Gaal, who has trained his fire on the media and sarcastically said losing to Derby could cost him his job for a fourth time this season.
But in the first half-hour and after the break, United showed the kind of swagger that has been missing in recent months.
Buoyed by more than 5,000 visiting fans, much of United's improvement centred around Anthony Martial, who captured some of his early season form with a superb performance.
Stationed on the left wing, the 20-year-old caused problems for Derby right-back Cyrus Christie and capped an effervescent display when he set up Mata for the final goal after skipping through the home side's defence.
With better control, the French forward could have scored either side of Rooney's clinical finish. Although the United striker appeared offside when he collected the ball, he turned inside before finding the top corner to move within six of Sir Bobby Charlton's club record 249 goals.
Derby challenge fades in second half
Derby have had their own problems in the Championship under manager Paul Clement, who was assistant to Carlo Ancelotti at Real Madrid, Paris St-Germain and Chelsea before he took his first step into management at the start of the season.
The former school teacher has overseen £25m of spending on players since taking over and, until Boxing Day, it seemed to be paying dividends with Derby on course for the Premier League.
But like last season, when the Rams faltered in the promotion push under Steve McClaren, their form has nosedived.
The hosts looked bereft of confidence as United began the game strongly, missing out on an opportunity to pressurise Van Gaal's side.
Yet once they pressed United higher up the pitch they began to give their expectant supporters something to cheer about.
Thorne's goal was well worked, and had Nick Blackman kept two efforts on target either side of the break, they might have caused real worry for the visitors.
However, Blackman and fellow winger Thomas Ince were well shackled by United's young full-backs Guillermo Varela and Cameron Borthwick-Jackson, and once Blind put the 11-time FA Cup winners ahead, Derby's challenge faded.
Man of the match: Anthony Martial
Manchester United manager Louis van Gaal: "It was maybe better than we had trained. We played at the right time against the resistance of Derby. We gave their goal away but at half-time I said that it was a good performance, keep it up and we will win. And we did.
"Daley Blind's goal? You have to sniff it and he sniffed it. It was a great goal. The Premier League is very important but the FA Cup is the greatest cup in England with a long and important history. We haven't won it for a long time so we dream of it. After two wins you can't say that we will win it."
Derby boss Paul Clement: "You're always worried about those quality players, like Juan Mata and Anthony Martial, off the back of our midfield. They were clinical, their movement and finishing was the difference.
"I thought we gave them a good game, it wasn't an easy game for them. We had some good passing spells and our goal was an excellent goal, a good finish from our holding midfielder - although I don't know what he was doing up there!
"I'm disappointed we didn't keep it tighter for a bit longer so we could go into the last 15-20 minutes with a chance. For a team who is lacking confidence they played well tonight."
Derby host Preston on Tuesday as they hope to get their Championship title challenge back on track, while Manchester United welcome Stoke to Old Trafford on the same evening.
- 37WarnockBooked at 54mins
- 34ThorneSubstituted forRussellat 76'minutes
- 15JohnsonSubstituted forHendrickat 76'minutes
- 22BlackmanSubstituted forCamaraat 59'minutes
- 1de Gea
- 28SchneiderlinSubstituted forCarrickat 74'minutes
- 8MataSubstituted forHerreraat 84'minutes
- Anthony Taylor
- Shots on Target
Second Half ends, Derby County 1, Manchester United 3.
Offside, Derby County. Jacob Butterfield tries a through ball, but Johnny Russell is caught offside.
Guillermo Varela (Manchester United) wins a free kick in the defensive half.
Foul by Tom Ince (Derby County).
Substitution, Manchester United. Ander Herrera replaces Juan Mata.
Goal! Derby County 1, Manchester United 3. Juan Mata (Manchester United) left footed shot from the centre of the box to the bottom right corner. Assisted by Anthony Martial.
Corner, Derby County. Conceded by Chris Smalling.
Foul by Wayne Rooney (Manchester United).
Richard Keogh (Derby County) wins a free kick in the defensive half.
Substitution, Derby County. Jeff Hendrick replaces Bradley Johnson.
Substitution, Derby County. Johnny Russell replaces George Thorne.
Offside, Manchester United. Juan Mata tries a through ball, but Anthony Martial is caught offside.
Substitution, Manchester United. Michael Carrick replaces Morgan Schneiderlin.
Attempt missed. Jacob Butterfield (Derby County) right footed shot from the centre of the box is high and wide to the left. Assisted by Abdoul Camara with a cross.
Corner, Derby County. Conceded by Guillermo Varela.
Foul by Morgan Schneiderlin (Manchester United).
George Thorne (Derby County) wins a free kick on the right wing.
Anthony Martial (Manchester United) wins a free kick on the left wing.
Foul by George Thorne (Derby County).