Pep Guardiola
Barcelona's coach Pep Guardiola attends a news conference at Joan Gamper training camp, near Barcelona. REUTERS

Barcelona open the defense of their Champions League crown against six-time winners AC Milan Tuesday evening.

The pick of this week's fixtures pits two of European football's most successful sides against each other with Barcelona bidding to become the first side to retain the Champions League crown in its modern incarnation.

Milan coach Massimiliano Allegri acknowledged that the gulf between the two sides is larger than it has been since the Rossoneri beat Barcelona 4-0 in the 1994 final.

"We're ready to play this game, a game of prestige and the first in the Champions League this season," he told reporters.

"We're going to try to make good start and be balanced in defence and attack. Right now, they are the team playing the best football, they have the highest quality - both individually and as a team - but we have our own quality, we're a good team.

"We're AC Milan, so we must play with courage. In football, you can never take anything for granted."

Allegri is without striker Zlatan Ibrahimovic for tonight's clash after the former Barca player was ruled out with an adductor injury.

"Unfortunately this morning at the end of training Ibra felt a twinge in his adductor and he has remained at home," Allegri said.

Brazilian forward Alexandre Pato is expected to partner Antonio Cassano for the Italian champions, who were held 2-2 at home to Lazio at the weekend.

Guardiola's side could only draw at Real Sociedad Saturday, despite holding a 2-0 lead at halftime.

The Barca coach is without Gerard Pique (calf) and Ibrahim Afellay (thigh) but Carles Puyol could feature after being included in the squad against Real Sociedad, following knee surgery. However, Alexis Sanchez will miss the next eight weeks after suffering a thigh injury on during Saturday's draw.

Prediction: Barcelona 3 - 0 AC Milan

Barcelona probable XI: Valdés; Alves, Busquets, Puyol, Adriano; Thiago Alcántara, Xavi, Iniesta; Pedro, Messi, Villa.

AC Milan probable XI: Abbiati;Abate, Nesta, Thiago Silva, Antonini; Gattuso, Ambrosini, Aquilani, Boateng; Cassano, Pato.