Ruud van Nistelrooy's departure from Manchester United was an object lesson in just how quickly even the strongest and most successful relationships can implode, but nearly two seasons later the acrimony has melted into admiration, the hurt supplanted by nostalgia.

Admiration, in particular, for the precocious winger who arrived at Old Trafford as an 18-year-old in 2003-04. That season, Van Nistelrooy scored 30 goals in all competitions, a tally Cristiano Ronaldo eclipsed last month. The player who annually vied with Thierry Henry to be England's most prolific striker is not about to contradict the consensus that the man who replaced him as United's primary goalscorer is capable of becoming the world's best footballer.

“Ronaldo's development has been the big change since I left. He stands out now. His improvement has been incredible,” Van Nistelrooy told Inside United, the official club magazine. “He came as a young lad and needed to improve. The manager gave him confidence by playing him all the time. By doing that he got the footballer he wanted. Ronaldo's now a great player. He makes the right decisions, taking the man on when he needs to and passing the ball at the right time. He does everything - he takes free kicks and corners. He leaves goal kicks to [Edwin] van der Sar, but I expect him to start taking them soon.”

Not that Ronaldo is the only young star in the United attack. Van Nistelrooy also offered high praise to Wayne Rooney: “He's excellent,” he said. “With his strength, pace and ability to beat a man he's always a threat. His finishing is good and he's still really young, so he'll only get better. United fans couldn't wish for any more at the moment than Rooney, Ronaldo, Nani and [Carlos] Tévez.”

The Holland striker believes that Ronaldo is in the same bracket as Kaká, the AC Milan superstar, and Lionel Messi, the Barcelona wunderkind. “If he carries on like this he'll be the best player in the world,” the forward said. “He's got everything.”

Van Nistelrooy's talents are less wide-ranging: he is a pure predator. In five years at Old Trafford he scored 95 times in 150 league games and became United's top scorer in the Champions League. Now 31, he has 12 goals in 20 league starts for Real Madrid, the La Liga leaders, this season, but faces six weeks on the sidelines after having surgery on his troublesome right ankle last month.

His switch to Madrid in the summer of 2006 was prompted by a collapse in communications with Sir Alex Ferguson, which was all the more surprising as it was the United manager's support and encouragement that clinched his transfer from PSV Eindhoven.

He was the leading goalscorer in the Netherlands in 1999-2000, after which a move to Manchester was agreed but put on hold at the eleventh hour when a medical raised concerns about a medial ligament problem. Days later, he suffered a serious knee injury in training. But Ferguson reassured Van Nistelrooy that he would make a full recovery and become a United player.

A year on, he did - their bond ensuring that Real's advances were spurned. By 2003, Van Nistelrooy's goalscoring ratio was so impressive that Ferguson suggested that one day the striker might earn his own statue at Old Trafford.

Three years on, the abiding image was of an immobile Van Nistelrooy, not immortalised in stone but sitting on the substitutes' bench, unused as United routed Wigan Athletic in the League Cup final. Ferguson continued to leave him out and the relationship between manager and player grew strained, stretched to breaking point for the final fixture of 2005-06, when Van Nistelrooy stormed out of Old Trafford after discovering in a pre-match meeting that he had been omitted from the team in favour of Giuseppe Rossi, a teenager. He asked for a transfer.

“I always thought I would finish my career at United, but sometimes things don't go the way you plan,” Van Nistelrooy said. “I had a wonderful time playing for United. My final months at the club were difficult, but I don't think of those when I remember my time at Old Trafford. I hold no grudges and sometimes I look back and laugh at the situation. I think we would act differently if the same situation repeated itself, but everyone has moved on.”

Van Nistelrooy keeps in contact with former team-mates. “United is a huge operation, but at the centre it feels like a family,” he said. “It's one of the biggest clubs in the world, but you don't feel lost.”

He compared Ferguson to Fabio Capello, the England manager. “As a United player everything is about training hard and winning things,” Van Nistelrooy said. “That's what the whole atmosphere at the club is about. It stems from the boss's authority. His will to win is so strong that it affects every aspect of the club.”