AC Milan's chances of dethroning the Champions League holder were dealt a major blow as center back Thiago Silva, who scored during a 2-2 draw between the two sides earlier this season, limped out of the Rossoneri's weekend clash with Roma.He has been ruled out for both legs of this quarterfinal, meaning coach Massimiliano Allegri will reshuffle the defense by welcoming back veteran Alessandro Nesta. Daniele Bonera will slot in at right back in place of the injured Ignazio Abate.Another key absence is the suspended midfield enforcer Mark Van Bommel, but Kevin-Prince Boateng and playmaker Clarence Seedorf are once again available and both could start, though Alberto Aquilani and Urby Emanuelson are also possibilities in the center of the pitch.Ibrahimovic, who tops the Serie A scoring charts with 22 goals, will lead the line against his former side. Robinho has recovered from an ankle problem and is likely to start alongside the former Inter star ahead of Maxi Lopez and Stephan El Shaarawy. Alexandre Pato has gone to the USA for treatment on his persistent muscle injuries and will not feature on Wednesday.Barcelona has a few worries of its own as defender Eric Abidal is scheduled for a liver transplant and another left back option, Adriano, is suffering from a thigh strain.Therefore, Carles Puyol is likely to be moved to left back with Javier Mascherano partnering Gerard Pique at center back. Coach Pep Guardiola could opt for a 3-4-3 system which he used against AC Milan earlier in the campaign.Dutch winger Ibrahim Afellay has been included in the matchday squad to continue his training, but will not take part in the actual fixture, following his recovery from knee ligament damage. Spain international striker David Villa remains a long term casualty.Which players will partner Lionel Messi upfront remains a mystery, but the two front-runners appear to be Cesc Fabregas and Alexis Sanchez with Pedro also in contention for a place in the starting XI. Did you know AC Milan has never lost the home leg of a European Cup quarterfinal tie, winning 10 and drawing four.The Serie A champion has conceded 11 goals, the most times of any side left in this season's competition.Current Milan players Ibrahimovic, Maxi Lopez, Van Bommel and Zambrotta have all had spells with the Blaugrana during their careers.Barcelona has already beaten AC Milan at San Siro this season, winning 3-2 on Matchday 5 during Group H action.Guardiola's side topped Group H with 16 points and the Italians were runners-up with nine points.The Catalans have won nine matches in a row in all competitions since a 3-2 loss to Osasuna in early February.