Barcelona starts the defense of its Champions League crown at home to European giant AC Milan on Tuesday unsettled by an injury to new signing Alexis Sanchez and a sloppy league display.

Pep Guardiola's well-established side has already scooped the Spanish and European Super Cups this term, and is aiming for a fourth Champions League title in seven years, but looked unusually vulnerable in La Liga on Saturday.

The Spanish champion frittered away a two-goal lead to draw 2-2 at Real Sociedad as complacency took over and its trademark passing game deserted it in the second half.

To rub salt in the wound, it lost new Chilean forward Sanchez to a torn hamstring that will keep him out for up to two months.

"(Our morale) isn't the best today," Barca midfielder Sergio Busquets said on Sunday. "But we will react, and we will do it well and quickly.

"Last year we also began with problems, losing to Hercules, but it's no good worrying about it. We have to take note of it and correct the mistakes."

Barca has often struggled after breaks for international soccer, suffering from the so-called "FIFA virus", and Guardiola's decision to start with Lionel Messi, David Villa and Andres Iniesta on the bench raised eyebrows on Saturday.

The team raced into an early lead with goals from Xavi and in-form new recruit Cesc Fabregas before going off the boil.

It is likely to be at near full strength for the Group H match against Milan and Guardiola could have Carles Puyol back from injury to bring some much needed poise and solidity back to the center of its defense.

Milan got off to a shaky start in Serie A, drawing 2-2 at home to Lazio on Friday, and had to come back from two down with goals from ex-Barca forward Zlatan Ibrahimovic and former Real Madrid striker Antonio Cassano.

Despite the team again being clearly the best-equipped side in Italy, few Milan fans expect the seven-time European Cup winner to threaten in the Champions League.

An early trip to the holder will be a gauge of its progress under Massimiliano Allegri after its elimination by Tottenham Hotspur in the last 16 last season.

Midfielder Gennaro Gattuso limped out of the Lazio clash but is suspended in Europe for four games anyway following his headbutt of Tottenham assistant boss Joe Jordan last season.