He told the Daily Mail: "I still do not understand the real motives of United and think the whole thing set a bad example for football. It is incredible that in the modern age - in the 21st century - there exists a situation where United will not sell to Liverpool.
"I only have good memories of the supporters and my team-mates but I would rather not give my opinion about the directors."
The 29-year-old Argentina left back insists that he is happy at Madrid but admits that he would love to knock United out of the Champions League. He added: "I really wanted to play for Liverpool but it is forgotten.
"During the summer I dreamed of playing in Anfield but the reality was different.
"I don't think often about revenge but it is certain that I want to play against Manchester United in the Champions League.
"That would be a big match and one without a clear favourite. But I would fancy Real.
"Manchester United never believed a big-name club would ever want me but I have proved them wrong and now I can't ask for anything more.
"This has just begun and there's a lot more to lay on the line."