Thiago Silva headed an injury time equalizer to give AC Milan a 2-2 draw against title holder Barcelona in their opening game of this season's Champions League on Tuesday.

Tournament favorite Barcelona trailed after just 24 seconds when Alexandre Pato dashed through Barcelona's half and coolly slid the ball under goalkeeper Victor Valdes to give the Italian champions the lead at the Camp Nou.

A determined Lionel Messi led the Spanish champions' response as the Argentina forward created Pedro Rodriguez's 36th-minute equalizer before David Villa curled home a long-range free kick in the 50th minute.

But Thiago leapt high to meet Clarence Seedorf's corner as a Milan team missing Zlatan Ibrahimovic and Robinho to injury spoiled the opening match of Barcelona's title defense in Group H. BATE Borisov and Viktoria Plzen drew 1-1 in the other group game to leave all four teams on one point.

Barcelona ended a disappointing night by learning that midfielder Andres Iniesta will be out for approximately four weeks due to a leg injury.

Barcelona's defense again let slip a lead in the final minutes, just three days after it surrendered a two-goal advantage to draw 2-2 at Real Sociedad in the Spanish league.

While Carles Puyol returned from injury to play as a second-half substitute, Pep Guardiola's use of midfielders Sergio Busquets and Javier Mascherano in the center of defense was exposed early as Pato sprinted past the pair untouched before slotting home a right-footed shot for the fifth-quickest goal ever in the Champions League.

Messi led Barcelona's fightback and kept goalkeeper Christian Abbiati busy throughout, including a low shot that preceded his free kick curling off the post in the 19th.

Barcelona dominated possession, but Pato troubled Barcelona's defense with his few touches as the Brazil striker made up for the absence of Ibrahimovic, who missed out on a return to the club he left last year. Pato slipped the ball between Busquets' legs and scurried through on goal before laying the ball off for Kevin-Prince Boateng to shoot high under pressure.

But Barcelona did better to keep the ball after that and Messi was causing problems as he forced Abbiati into a save on the half hour mark before setting up Pedro's easy finish. Messi flicked a ball between Alessandro Nesta and Ignazio Abate toward the goal line and reached it first to cross into the box for his unmarked teammate.

Cesc Fabregas replaced Iniesta in the 38th after the Spain international pulled up limp after a run, and the home side's talented midfield completed nearly seven times as many passes as its opponent, with Abbiati often on the receiving end of such buildups - including in the 48th when he flung himself across goal to tip Xavi Hernandez's shot out.

Abbiati was again flying though the air two minutes later but could not reach Villa's free kick as it found the top corner of the net after evading the defensive wall.

Puyol came on in the 67th for his first appearance since May, but even the captain's return did not provide extra stability with Gerard Pique already missing. Barcelona is also missing Chile forward Alexis Sanchez to injury.

Milan managed to test Barcelona 'keeper Victor Valdes once more before Thiago scored the equalizer with a well-guided header.