Beckham and Rooney will line up in opposition when Milan face United in the Champions League last-16 on Tuesday, and the former England captain told BBC Sport the 24-year-old is now "without a doubt" on a par with the Brazilian ace, who was named the best player in the world in 2004 and 2005.
"I've always said Wayne is one of the best goalscorers in football," said Beckham, who will face his old side for the first time since his acrimonious departure in 2003.
"He is at a great club like Manchester United and he's doing what he does best - scoring goals."
Rooney has enjoyed his finest goalscoring season to date, netting 23 times in 32 appearances for United this term, and Beckham believes his England colleague's form will make it difficult for Milan when the two sides meet at San Siro.
"It's one of the biggest games for me personally. But the most important thing is the team going through and beating United which I hope we do," he said.
"United are on fire, they're playing well. Wayne is scoring goals like he can. It's going to be a tough game."Keep up to date with Serie A and Italy news with Goal.com's Italy page