They will travel to Wembley on 1 March as favourites to beat Tottenham and add the Carling Cup to their haul of silverware, while an intriguing clash with Jose Mourinho's Inter Milan awaits in the last 16 of the Champions League.
Derby have scored more goals against the champions than any other side this season having found the net four times over three matches, but Clough believes there are worrying signs for United's future opponents.
"We have played against them three times this season now and this was the best they played," he added.
Ferguson said he was confused by Cristiano Ronaldo's belatedly disallowed goal in the emphatic win at Pride Park.
Giggs discusses the disallowed goal with referee Alan Wiley
Ronaldo raced clear to score but the assistant referee flagged offside after learning from the referee that Ryan Giggs had flicked the ball on.
"I got an explanation but I'm not sure whether it works," said United's boss.
"It should've been given straight away. Our keeper threw the ball so he was off [whether or not Giggs touched it]."
The decision, which came with United leading 1-0, did not throw Ferguson's much-changed side off their game and Darron Gibson soon made it 2-0 with a fine volley.
Another United youngster, Danny Wellbeck, wrapped up the scoring and Ferguson was quick to highlight how important the depth of his squad has been in recent weeks and will need to be in the weeks to come.
He added: "I've been saying for some weeks with all the injuries, it's the squad that's been carrying us through difficult games and they have proved it again.
"With the game on Wednesday [against Fulham in the Premier League] in mind I made some changes and it worked out well."
Clough paid tribute to his side's resilience after they fought back well from 3-0 down early in the second half.
"It probably puts it in perspective when you consider the side we're playing against," he said.
"They could pretty much score at will.
"But we are delighted with the second-half performance. We had a good spell after United's third goal and we might have got it back to 3-2, but overall we're pleased even though we lost 4-1.
"We kept going, we kept playing when it would have been easy to roll over."
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