The Rossoneri bossed the opening 45 minutes in Manchester and led at the break with Kaka's double cancelling out Cristiano Ronaldo's early header.
But United stormed back after the interval and Wayne Rooney's two goals, the second of which came in injury time, turned the match around.
Kaka said: "You only beat Manchester United if you play as we did in the opening half at Old Trafford.
"We have to show character, personality and have confidence.
"Manchester United are a strong team, they have a fast counter-attack, they have strikers that make great moves up front. On top of that, they are a young side."
Kaka was impressed by the atmosphere at Old Trafford and has called on a sold-out San Siro to roar their favourites to victory on Wednesday night.
"The game at Old Trafford was a great show," he added.
Atmosphere
"But I am convinced that the return leg will be equally beautiful, or even better.
"I was pleasantly surprised by the atmosphere at Old Trafford.
"At half-time, their (Manchester United) fans didn't stop cheering their team despite the fact that we were 2-1 up.
"I am certain that our supporters will show the same spirit on Wednesday."
It is the second round in succession that Milan have gone into the second leg scrapping to stay in the competition.
In the last eight they could only manage a 2-2 draw at home to Bayern Munich in the first leg.
But goals from Clarence Seedorf and Filippo Inzaghi at the Allianz Arena saw Carlo Ancelotti's side grab a 2-0 lead they did not relinquish.
Kaka is hoping the experience of that success will stand them in good stead against United.
He said: "We will need to give a mighty performance just as we showed against Bayern Munich.
"But this time we have an advantage, we are playing at home and it will be a great show."
Milan go into the game in good form. They are unbeaten in 2007 and, at the weekend, rose to third in the Serie A standings.
How do you think they game will go tactically? Have your say.
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Demo, Glossop (30/04/2007 at 16:09)