Beckham not assured start against United
The midfielder has been on the bench for the last two league games and the return from injury of forward Alexandre Pato makes it even less likely the 34-year-old will play.
However, coach Leonardo knows how much it means for Beckham to face Alex Ferguson's men for the first time even if the former England captain is trying to stay cool.
"I always said when I came to the club that I never expect to start any game," Beckham, who won the 1999 Champions League with United before leaving in 2003, told reporters.
"I've started the first three or four games. Obviously last week I didn't play and then this week I came on so no I'm not frustrated at all."
Milan, who have never lost to United over two legs and have won their four encounters in Italy, snapped a four-game winless run in Friday's 3-2 victory over Udinese watched by Ferguson.
On-loan Beckham came on with 15 minutes left and played one dreadful pass which almost led to an Udinese goal.
Worse for Milan was a thigh muscle injury which forced centre back Thiago Silva to go off.
The Brazilian is battling to shake off the problem for Tuesday and partner fit-again Alessandro Nesta at the back while striker Marco Borriello is a doubt and Klaas Jan Huntelaar could again deputise, having scored twice against Udinese.
Ronaldinho had one of his better games on the left of the front three but the return of Brazilian compatriot Pato, out since the start of the year with a thigh problem, will give Milan more pace on the right.
English champions United have fewer injury worries but centre back Nemanja Vidic has stayed at home because of a nerve problem in his leg and Beckham's ex-team mate Ryan Giggs is out with a broken arm.
"It's a big shame that Giggsy is out. To play on the same field as Giggsy against him would have been really nice. Hopefully he'll be fit soon and may make the second leg (on March 10)," Beckham added.
Last season's runners-up United, who lost to Milan in the 2007 semi-finals before winning the trophy for the third time in 2008, have had a free weekend because of their FA Cup exit and also welcome back Rio Ferdinand from a domestic suspension.
Striker Wayne Rooney is in the form of his life but is taking nothing for granted against the seven-times champions.
"Milan have a number of players who can score goals, particularly from set pieces, so I think we'll have to be wary of that," Rooney told United's website.
Milan: 1-Dida; 20-Ignazio Abate, 25-Daniele Bonera, 13-Alessandro Nesta, 77-Luca Antonini; 8-Gennaro Gattuso, 21-Andrea Pirlo, 23-Massimo Ambrosini; 7-Alexandre Pato, 11-Klaas Jan Huntelaar, 80-Ronaldinho
Manchester United: 1-Edwin van der Sar; 6-Wes Brown, 23-Jonny Evans, 5-Rio Ferdinand, 3-Patrice Evra; 25-Antonio Valencia, 24-Darren Fletcher, 16-Michael Carrick, 17-Nani; 10-Wayne Rooney, 9-Dimitar Berbatov.