United manager Alex Ferguson is without eight players - including three first-choice defenders - for the semifinals of the Champions League at Old Trafford.
Rio Ferdinand limped off in Saturday's 1-1 draw with Middlesbrough after aggravating a groin injury, joining fellow defenders Nemanja Vidic, Gary Neville and Mikael Silvestre on the sidelines.
"We're down to the bare bones," Ferguson said. "It's not a time for feeling sorry for ourselves. It's time to show what we are made of.
"It's been well documented in the history of our football club that we do it the hard way anyway."
It should be made easier if winger Cristiano Ronaldo is at the top of his game, tormenting defenders.
The Portugal winger is full of confidence after scoring his first European goal for United in the 7-1 quarterfinal rout of the Rossoneri's Serie A rival AS Roma.
On Sunday, he became the first player in three decades to win two player of the year awards voted for by his fellow Premier League players.
"At this moment, Cristiano Ronaldo is on top form," Milan defender Alessandro Nesta said. "But he can be stopped by being organized, by attacking him as soon as he gets on the ball."
While United is struggling to field a team, Nesta was one of six players rested in Saturday's 3-1 win over Calgiari in the Serie A.
Another was Brazil midfielder Kaka, who is the Champions League's top scorer with seven goals.
"We arrive in perfect condition, relaxed and convinced," coach Carlo Ancelotti said. "They have some problems, but in games like these everything is solvable with great determination and concentration."
Ancelotti's biggest concern is over Brazil goalkeeper Dida's shoulder injury. He traveled with the squad to Manchester on Monday, but remains in doubt for Milan's fourth semifinal match in five years.
Milan's most recent victory over the Red Devils came in the second round in 2005.
United has lost just once in 29 games at Old Trafford in the Champions League, but Milan has conceded just one away goal in Europe this season.
"Playing at Manchester is hard - they have an amazing public which is easily aroused," Nesta said. "The atmosphere in general will be 50 percent of the battle."
Old Trafford was the venue for Milan's sixth European Cup victory - in 2003 against Juventus on penalty kicks - while United hasn't lifted the trophy since 1999, when it also beat the Turin club.