Now Derby captain, Rooney is United's all-time top goalscorer after a trophy-laden 13 years at Old Trafford between 2004 and 2017, and he has had a big impact in both a playing and coaching capacity since arriving at Pride Park in January.
"Of course Wayne is very important, and maybe even more so in this game because he knows Manchester United like no-one else," said Cocu. "He knows the players, the level of performance, what is needed in those big games. So yes, it will be extremely important for us".
Rooney's arrival has helped the Rams pull clear of the Championship relegation zone and Cocu believes the focus on the former England captain helps ease the pressure on some of the younger members of his inexperienced squad.
"I think it could be important for the rest of the team," he said.