Rooney scored the Championship side's fourth goal from the penalty spot in Tuesday's fourth-round replay.

The win means Derby will host Manchester United in the week commencing 2 March.

"I'm sure he will be ready," said Cocu.

"He's (Wayne Rooney) going to face the club where he had a wonderful career so of course it is special."

Rooney's influence clear to see

It will be the first time that Rooney - United and England's all-time top scorer - has faced his former side since playing for Everton in a 2-0 defeat at Goodison on 1 January, 2018.

Playing as a deep-lying midfielder, the 34-year-old has been a key player for Derby since joining Cocu's side at the start of January.

The Rams have won five and lost just one of the eight games he has been involved in, while his latest goal was his third in as many games for the club.

"He comes back from the States, joins us and now plays his old club where he had a great career," added Cocu.

"It will be a beautiful game not just for Wayne, but everyone at the club. It's a great game to look forward to and we will also try to win it, against a strong opponent."

Rooney played a pivotal role in Derby's 1-0 win against Premier League side Crystal Palace in the third round and was once again involved in most of this game's key moments.

It was from his corner that the hosts took the lead as David Cornell failed to deal with the delivery and Andre Wisdom pounced on the loose ball to score his first goal for Derby and his first since his debut for Liverpool in a Europa League tie against Young Boys eight years ago.

Apart from a brief spell in the first half, Derby were in complete control of the first half and deservedly added a second when Duane Holmes was put through by Scott Malone, drew out the keeper before side-footing home.

Out of nowhere, however, the visitors halved the deficit when the Derby defence was caught half asleep soon after the start of the second half with Nick Adams driving the ball in from a decent cross.

But Derby restored their two-goal advantage when Jack Marriot headed powerfully at goal and, although David Cornell got a strong hand to it, the ball had just gone over the goalline.

Northampton pressed to try and set up a tense finish but hopes of comeback were ended when a foul on Tom Lawrence presented Rooney with the chance to get on the scoresheet and he made no mistake with confident spot-kick into the bottom corner.

The visitors then converted a penalty of their own when Andy Williams was brought down inside the box in the final 10 minutes and, although Sam Hoskins converted, it was to be nothing more than a consolation for the visitors.

'We needed to make them squeal'

Northampton manager Keith Curle felt they gave Derby too much respect.

"In the first half I didn't think we turned it enough into a cup tie," he said. "We needed to upset their rhythm.

"They pass and move very well and it can make it difficult. When you are playing against good players you need to slow them down a bit.

"Without being too crude, we needed to make it more of a cup tie, we needed to make them squeal a little bit.

"Overall, we came and we competed. My players will do that because that's the demands I put on them. The effort and commitment, when someone puts the shirt on, that's there."