Tuesday's last-16 first-leg match at the San Siro will be an emotional occasion for Beckham, who joined United as a youngster and won the 1999 Champions League there before leaving in 2003.
"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," Beckham told reporters.
"United are on fire, they're playing well. Wayne is scoring goals like he can. It's going to be a tough game."
Asked if Rooney was now comparable to twice world player of the year and Milan team mate Ronaldinho, Beckham said: "Without a doubt, I've always said Wayne is one of the best goalscorers in football.
"He's proving that and he is at a great club like Manchester United so he's doing what he does best - scoring goals."
Beckham, not assured of a start on Tuesday, came on as a substitute in Milan's patchy 3-2 Serie A victory over Udinese on Friday which ended a run of four games without a win.
Despite the retirement of Paolo Maldini and the sale of Kaka to Real Madrid, Beckham still believes underdogs Milan can eliminate United in his first clash against Alex Ferguson's men.
"Obviously there are a couple of players not here...but the team is just the same, they've got this great team spirit within the squad and that's definitely not changed," said Beckham, who is on his second loan spell at Milan from Los Angeles Galaxy.
"But I think we are just as strong and when we play well we are a very good team."
Beckham, who feels in good form and fitness having returned to Milan to safeguard an England World Cup place, is looking for a strong home performance from Leonardo's side on Tuesday.
"It's always good to get a good result in the first game especially when you are at home," the 34-year-old said, adding that the emotion of the night would not get the better of him.
"I've never had to do it before like this. I don't think it'll be a problem. I've played in many games."
Beckham refused to comment about John Terry being dropped as England captain after allegations about his private life.
"To be honest so much has been said about the whole situation I don't even think I need to comment on it. It's nothing to do with me," the former national skipper said.
