"I do not expect any problems and probably the negotiations are going to be finished very soon," the 27-year-old Dutch international told MUTV on Thursday.
"I want to stay here for a long time, that is why I am so happy that everything is going so well."
Van Nistelrooy has scored 97 goals in 125 appearances for the Old Trafford club since arriving in 2001 from PSV Eindhoven for 19 million pounds ($33.85 million).
"It is fantastic to be part of a club with such big names and huge tradition," he said. "I just want to carry on doing my best."
United manager Alex Ferguson is reportedly trying to buy a new striker to provide cover for Van Nistelrooy, whose fitness is crucial to the team's chances of winning the Champions League and their ninth premier league title in 12 seasons.
United are one point ahead of Arsenal at the top of the domestic standings halfway through the season and will play Portuguese club Porto in the last 16 of the Champions League
