Defending champion Barcelona kick off their title defense on Tuesday when they host AC Milan at Nou Camp in the first match of 2011 Champions League group play.

Perhaps the most heavily anticipated match in group play, both clubs boast a collection of world class players. Lionel Messi, perhaps the best player in the world, will be available for Barcelona, and so will Carles Puyol who is expected to return after rehabbing from knee surgery.

Eric Abidal will play centre back in place of Gerard Pique, who will be sidelined for the match, and Chilean forward Alexis Sanchez is injured, as well.

AC Milan are led by Brazilian forward Alexandre Pato, and is joined by former Barcelona standouts Mark van Bommel and Gianluca Zambrotta. However, another former Barcelona star, Zlatan Ibrahimovic, will miss the match due to injury. Also inactive for AC Milan will be former Real Madrid striker Robinho.

I Rossoneri are coming off an exciting 2-2 result with Lazio, in their first Serie A match of the year. Massimiliano Allegri's squad will need a better defensive effort against a Barcelona attack that features Xavi Hernandez and Andres Iniesta, possibly the best midfield tandem in the world.

AC Milan have a strong midfield, too, and could take advantage of some holes in Barcelona's lineup. Pep Guardiola's squad have had some recent defensive lapses. On Saturday, Real Sociedad rallied from a two-goal deficit to draw  2-2 with Barcelona.

We have to do better and prepare better, said Guardiola after the Real Sociedad match.

Though Barcelona often appear invincible, in the Super Cup against Real Madrid, Abidal struggled to connect on passes, and to maintain possession, and while the entire Barcelona squad at times looked out of sorts.

AC Milan, with their potent attack, might be able to exploit some of Barcelona's defensive weakness, particularly with Pique out of the starting eleven.

Barcelona and AC Milan are expected to advance out of the group at the expense of BATE Borisov of Belarus, and Viktoria Plzen of the Czech Republic, who meet Tuesday in Prague.