The ex-Netherlands international is eagerly anticipating the clash between two of his former clubs, and believes minor details will be decisive in deciding the contest
The former Netherlands international, who enjoyed spells with both European giants, feels there is no clear favourite ahead of next week's first leg, and has stressed that small details could decide the outcome of 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 was quoted as saying by 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.