The former Netherlands international, who enjoyed spells with both European giants, feels there is no clear favourite ahead of next week's round-of-16 first leg, and believes Robin van Persie and Cristiano Ronaldo will wield considerable influence over the tie.

"There is no favourite. For me the key will be the form of Cristiano and Van Persie. One of them will make the difference," Van Nistelrooy told Marca.

"I think the tie will be decided by minor details. Both teams are strong in attack.

"Real Madrid have improved a lot in the last few weeks. Apart from the game in Granada the team has been strong; Real played well against Barca and rise to big occasions.

"The Champions League is more of an obsession for Real Madrid than Manchester United."

The first leg takes place at the Santiago Bernabeu on February 13.