The 32-year-old Dutchman had surgery on his right knee this week in the United States and told Friday's Marca newspaper that his career would have been over without it.
"If they hadn't put me in the operating theatre football would have been finished for me," he said in an interview with the paper in Colorado where the operation was conducted.
"It's a long time to be out but there was no other way," he added. "The injury is serious."
Van Nistelrooy was treated by Richard Steadman, the surgeon who operated on the same knee in 2000 while the player was at PSV Eindhoven.
"It's true I am 32 but eight years ago I was in a similar situation so I know what to do now," van Nistelrooy told Marca.
"The recuperation will be very hard but I will manage it. I still have a lot of goals to score in the Real Madrid shirt and many objectives to achieve."
Van Nistelrooy damaged the knee during Real's 2-1 Champions League defeat at Juventus last month. He returned for the 2-0 home defeat by the Italians, but a scan appeared to show an aggravation of the injury.
"It's one of the most difficult moments of my career, that's clear," he told Marca. "But there are two options: give up or fight. And I am going to fight."
