The former Manchester United and Real Madrid striker is refusing to guarantee he'll be around for Euro 2012 despite recently making his Holland comeback.
Van Nistelrooy has made a lively start to the season at Hamburg, scoring six in five games, but he fears his incarnation of a centre forward is a dying breed in world football.
Van the man: Ruud van Nistelrooy has suggested that the current campaign with Hamburg may be his last as a professional
'I don't have long-term plans anymore,' he told Frankfurter Allgemeinen Sonntagszeitung. 'I have to listen to my body, so I can't say now that I'll be playing in 2012.
'I look at everything from year to year, see how it is going and then decide whether to carry on. I'm aiming to play consistently and well this season.
'Perhaps I'll carry on, perhaps not. I've no idea how long I have left to play.
Devilishly good Red: Van Nistelrooy was the most feared striker of his day in the Premier League
'If Hamburg qualify for the Champions League it would be nice, but when I feel that it is time up for me then I have to go - in or out of the Champions League.'
Admirers in Germany still rate Van Nistelrooy as among the best strikers in the world, but he believes the upper echelon is inhabited by a different type of player.
After scoring 75 goals in 91 games for PSV Eindhoven, the 6ft 2in centre forward took his killer instinct to Old Trafford in 2001, hitting 150 in in 219 games and rarely finding the net from outside the 18-yard box.
'Is Lionel Messi a striker?' he ponders. 'He actually embodies a complete forward. He scores goals, he makes them, he dribbles. Just like Cristiano Ronaldo.
'This is a completely new style, neither is like me, a typical No 9 who stands up top and always wants to be in the box when something happens. These strikers I don't see so often.
'In the box I can use my best qualities. Everything must come together in less than a second. Where is the defender? Where is the free space? Which of my team-mates has the ball? Marcell Jansen does different things to Eljero Elia, so I must also react differently. A lot has to do with intuition, with the instinct.
'As a striker you need a certain liveliness. Otherwise you're not getting away from your opponent. You always have to be a bit faster, be smarter and do things differently than they would expect.
'It's not just about experience, your body has to join in. If I ever have an idea in mind, but I can no longer physically do it, then perhaps I must think that's it.'
To Holl and back! Veteran striker Van Nistelrooy handed international recallSir Alex ready for 700th Premier League match - he's in a league of his ownFOLLOW OUR DEDICATED WORLD FOOTBALL TWITTER SERVICEAfter Antonio Valencia's horror injury, Sportsmail recalls the worst bad breaks suffered by Premier League players