Dennis Wise headed off the line but van Nistelrooy doubled United’s lead from the penalty spot after David Livermore had brought down Ryan Giggs.
Van Nistelrooy then tapped home a driven Giggs cross to ensure United’s disappointing season ended on a high.
Both Keane and Wise overcame their injury problems to make the final.
But Ronaldo, often pilloried by critics for over-playing this season, was the game’s outstanding footballer. His trickery and incisive runs hurt Millwall time and again, with full-back Robbie Ryan bearing the brunt of the youngster’s repertoire of skills.
Millwall settled quickly, enjoying plenty of early touches as United struggled to get the ball.
But Sir Alex Ferguson’s team quickly unveiled their attacking threat. Ronaldo, wearing gold boots, crossed from the right after four minutes and van Nistelrooy got across the Millwall defence only for his diving header to drift narrowly wide of Marshall’s goal. More inventive work from Ronaldo again exposed Millwall shortly afterwards.
The 19-year-old turned Ryan inside and out and crossed, but after the ball broke to Paul Scholes the England midfielder failed to make contact from six yards. United continued to press and Marshall made a brilliant one-handed save to deny Keane, who thought he had opened the scoring with a swerving long-range strike.
Three minutes from the break Marshall only half saved a shot from Ronaldo and Darren Ward showed great positioning to hack the ball off the line.
But Ronaldo’s superb first-half display was eventually rewarded with a goal. United manoeuvred the ball crisply from left to right and Ronaldo attacked the far post to head Gary Neville’s tempting cross past Marshall.
The goal was slightly unfortunate on Millwall, who had blunted United’s attacking threat for large periods while enjoying moments of their own.
A cleverly taken short corner caught United flat in defence and Tim Howard had to punch the ball off Tim Cahill’s head, while Paul Ifill broke down the right but had his shot blocked.
But their situation could have been even worse at the break after player-manager Wise repeatedly tangled with Paul Scholes. Referee Jeff Winter however opted for a cautionary word with both players and ensured Wise left the field without further harming his team’s cause. (BBC)
