“It was top-class, as good as they come,” said  Lampard.

“For a goalscorer to get across your man at pace and to guide it into the far corner as he did is a really difficult skill, and I didn’t realise how good it was until I saw a replay.

“He’s a boy who scores goals and I don’t want to give him too much because he needs to keep scoring, and I don’t want him to get a big head!

“But he looks lean, he looks fast, committed to defend for the team and, more than that, he can get us goals.

“I’m demanding of centre forwards because I want them to work for the team, they can’t just think about goals. Jack was probably a bit short on fitness when he got here so that took a few weeks, but he loves practising and he’s improving because the more you practice, the more you improve.

“He wants to practice after training every day and sometimes I have to hold him back . Other days I don’t because I love seeing that, and to see him do it makes me realise that if he keeps doing it he’s got every opportunity to have a big future.”

Derby caught United cold from the kick-off when a six man move was finished from 15 yards by Bryson with only 18 seconds played.

But United regrouped to dominate possession and equalised in the 41st minute when John Fleck went through Derby’s defence on the left and cut the ball back for Chris Basham to fire into the roof of the net.

It was a different story after the break as Derby started to stretch United and they went close several times before Marriott won it with a quality finish in the 77th minute.

Craig Forsyth whipped the ball to the near post and Marriott got in front of a defender to steer a shot across Dean Henderson.

United almost replied when Scott Carson dropped a cross and Jack O’Connell’s header was cleared off the line by Mason Mount.

Blades manager Chris Wilder said: “It’s a great result for Frank and Derby and a poor one for us.

“It was a tight game between two really good sides and they found their moment..

“Derby were better than us second half. We lost control of the game a little bit but I thought we were really good in the first.

“We were really good and should have found that last bit to go in at half-time up in the game, but we didn’t and they came back.

“I’ve no complaints about the second half and the result overall because if you defend like that and don’t put teams to bed the result can go either way.”

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