Milan's failings at the back were ruthlessly exposed by Juventus in last Sunday's 4-2 defeat which left them nine points behind leaders Inter Milan in joint third.
They were also on show on Wednesday, when they were held 2-2 by VfL Wolfsburg and missed the chance to go into the draw for the knockout phase of the UEFA Cup as group winners.
Coach Carlo Ancelotti is short both of defenders and the midfielders whose job it is to provide a filter for the back line.
Philippe Senderos came off early in the Wolfsburg game and media say he is likely to join fellow centre backs Daniele Bonera and Alessandro Nesta in the sick bay with the latest of a series of injuries he has suffered since joining Milan in the close season.
Full back Gianluca Zambrotta and midfielder Massimo Ambrosini are suspended this weekend.
Gennaro Gattuso will take six months to recover from knee surgery, while Mathieu Flamini is a fitness doubt for Sunday after missing the Juventus game, as is Andrea Pirlo, who came off at halftime against Wolfsburg because of a thigh pain.
"Things have been complicated by injuries and this penalises us a lot because we are forced to play with lineups that are not very tried and tested, not very compact," Ancelotti was quoted as saying by the club website.
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"I have to field players out of position.
"This team have quality and manage to score despite the difficult moment, but we have more problems in defence."
Ancelotti said he hoped to have some players back from injury, including Brazil playmaker Kaka from a groin problem, against fast-sinking Udinese, who have not won since October in Serie A.
In January, Milan will also be strengthened by England winger David Beckham, who is coming on a short-term loan from Los Angeles Galaxy and will be presented at the San Siro on Saturday.
But Gattuso's dynamism in the centre of the pitch will be badly missed and Ancelotti admitted that more reinforcements may be necessary during the January transfer window.
"If a midfielder arrived on the market, it would certainly be useful," he was quoted as saying by La Gazzetta dello Sport.
Champions Inter visit Siena on Saturday while Juventus, who are six points behind them in second, take on Atalanta in Bergamo on Sunday.
Napoli, who share third place with Milan, visit third-from-bottom Torino on Sunday.
