Pep Guardiola's well-established side have already scooped the Spanish and European Super Cups this term, and are aiming for a fourth Champions League title in seven years, but looked unusually vulnerable in La Liga on Saturday.
The Spanish champions frittered away a two-goal lead to draw 2-2 at Real Sociedad as complacency took over and their trademark passing game deserted them in the second half.
To rub salt in the wound, they 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 told a news conference 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 have often struggled after breaks for international football, 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 some eyebrows on Saturday.
They raced into an early lead with goals from Xavi and in-form new recruit Cesc Fabregas before going off the boil.
They are 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 centre of their defence.
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 their team again being clearly the best-equipped side in Italy, few Milan fans expect the seven-times European Cup winners to threaten in the Champions League.
An early trip to the holders will be a gauge of their progress under Massimiliano Allegri after their elimination by Tottenham Hotspur in the last 16 last term.
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.
New defender Philippe Mexes is also suspended and injured.
Mark van Bommel was a doubt before the Lazio clash but came on for Gattuso and impressed. He will now want to do the same against his former club.
"We must improve defensively as a team and return to not conceding," Allegri said.
Barcelona: 1-Victor Valdes; 2-Daniel Alves, 5-Carles Puyol, 14-Javier Mascherano, 22-Eric Abidal; 16-Sergio Busquets, 6-Xavi, 8-Andres Iniesta; 7-David Villa, 10-Lionel Messi, 17-Pedro.
Milan: 32-Christian Abbiati; 20-Ignazio Abate, 33-Thiago Silva, 13-Alessandro Nesta, 77-Luca Antonini; 23-Massimo Ambrosini, 4-Mark van Bommel,
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