Match ends, Manchester United 2, Derby County 0.
Manchester United left it late to beat Derby County in an entertaining FA Cup third-round game at Old Trafford.
A replay looked on the cards until Romelu Lukaku laid the ball off to Jesse Lingard, who took a touch and sent a 20-yard shot on the bounce into the top corner.
And half-time substitute Lukaku sealed the win after playing a one-two with Anthony Martial to cap off a counter attack.
United striker Marcus Rashford had hit the post in both halves - with a close-range header that he should have buried and a fierce shot across goal.
Rams keeper Scott Carson was excellent, making eight saves, including fine stops to keep out Juan Mata and Paul Pogba free-kicks.
Derby, second in the Championship, played well and Tom Lawrence forced a good diving save from Sergio Romero.
Mourinho's strong team pays off
After losing to Derby's Championship promotion rivals Bristol City in the Carabao Cup, United boss Jose Mourinho wanted to make sure there was no repeat and named a strong line-up, even though he made five changes.
Sitting 15 points behind Manchester City in the Premier League, and not among the favourites for the Champions League, the FA Cup is their best chance of winning a trophy this season.
They dominated, with Pogba, Mata and Rashford forcing saves from Carson.
But the big chance of the first half came when Henrikh Mkhitaryan's cross found Rashford, who was unmarked and only four yards out but somehow he headed against the post.
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Mkhitaryan was poor, that cross aside, and was replaced at half-time by Lukaku, six days after the Belgium striker left the pitch on a stretcher with a head injury against Southampton.
It was more of the same again in the second half, with Mata having a shot parried, Lukaku having a follow-up blocked and then Pogba firing wide.
Rashford again hit the post, this time from a tight angle across goal from Lingard's pass.
And with six minutes to go, Lingard gave United the lead when he lashed home from Lukaku's lay-off.
Derby pushed for an equaliser, sending men up for a corner, and Pogba's 60-yard clearance found Lukaku on the halfway line. He ran towards goal and played a one-two with fellow substitute Martial before smashing the ball through Carson.
Plenty of encouragement from Derby
While United deserved their win, Derby played their part in a good game. Like the Red Devils, they made plenty of changes - six - but still played a strong team.
Left-back Marcus Olsson had plenty of success down the wing in the first half and forced Romero into a save from his header.
Tom Huddlestone had a shot blocked, with the United keeper possibly beaten, and Lawrence produced another save from the Argentine.
Derby could probably have done without a replay and they now focus on their Championship promotion push. They sit second in the league, two points above third-placed Cardiff.
If they can stay there, they will be back at Old Trafford in the Premier League next year.
Manager reaction - 'Championship teams are Premier League level'
Manchester United boss Jose Mourinho to BBC Sport: "I was feeling that we were close to scoring and we never stopped trying. But when you get past 80 minutes and the goal hasn't come, you fear the second match. But the team kept trying so I knew the goal could arrive.
"It was a great shot by Jesse Lingard in the top corner, he's in a moment of confidence where things go well for him. He's a good professional and a fantastic kid in the dressing room.
"Derby defended very well. They changed players as the Championship is important to them. Scott Carson played well and the post played well for them. They were close to a replay - but they didn't need it.
"The Championship is coming in a direction where they are Premier League level, they just play in another competition."
Derby boss Gary Rowett to BBC Sport: "The lads worked incredibly hard and had to against a star-studded line-up. I think they showed us respect with the line-up they put out.
"We stuck at it and rode one or two moments of luck and started to get ourselves into the game. After weathering the moments out then you could hang on for the draw or nick one.
"We spoke about having no pressure but really for me it was a test to see who could handle it.
"We moved the ball with confidence and control. Of course it was fleeting but when we exerted pressure I thought we caused Manchester United uncomfortable moments.
"I think it was a more important game for Manchester United than it was or us. We're second in our league and that's our only focus really."
Manchester United now have 10 days until their next game - a home Premier League match with Stoke City on Monday, 15 January (20:00 GMT). Second-placed Derby are away to Birmingham in the Championship next Saturday at 15:00 GMT.
- 8MataSubstituted forMartialat 67'minutes
- 22MkhitaryanSubstituted forLukakuat 45'minutes
- 19RashfordSubstituted forFellainiat 80'minutes
- 40Castro Pereira
- 7RussellSubstituted forHansonat 78'minutes
- 10LawrenceSubstituted forBennettat 81'minutes
- 17WinnallSubstituted forVydraat 67'minutes
- Kevin Friend
- Shots on Target
Second Half ends, Manchester United 2, Derby County 0.
Goal! Manchester United 2, Derby County 0. Romelu Lukaku (Manchester United) right footed shot from the centre of the box to the bottom right corner. Assisted by Anthony Martial following a fast break.
Attempt missed. George Thorne (Derby County) header from the centre of the box misses to the right. Assisted by Marcus Olsson with a cross.
Victor Lindelöf (Manchester United) wins a free kick in the defensive half.
Foul by Mason Bennett (Derby County).
Attempt missed. Tom Huddlestone (Derby County) right footed shot from outside the box misses to the left. Assisted by Andreas Weimann.
Marouane Fellaini (Manchester United) wins a free kick on the right wing.
Foul by Mason Bennett (Derby County).
Goal! Manchester United 1, Derby County 0. Jesse Lingard (Manchester United) right footed shot from outside the box to the top right corner. Assisted by Romelu Lukaku.
Offside, Derby County. Tom Huddlestone tries a through ball, but Richard Keogh is caught offside.
Foul by Victor Lindelöf (Manchester United).
Mason Bennett (Derby County) wins a free kick on the left wing.
Substitution, Derby County. Mason Bennett replaces Tom Lawrence.
Foul by Marouane Fellaini (Manchester United).
George Thorne (Derby County) wins a free kick in the defensive half.
Substitution, Manchester United. Marouane Fellaini replaces Marcus Rashford.
Attempt blocked. Matej Vydra (Derby County) right footed shot from outside the box is blocked. Assisted by Andreas Weimann.
Substitution, Derby County. Jamie Hanson replaces Johnny Russell.
Attempt missed. Paul Pogba (Manchester United) right footed shot from the centre of the box misses to the left.