Milan coach Massimiliano Allegri said the absence of the volatile Swede, who fell out with Barca coach Pep Guardiola and was offloaded at the end of the 2009/10 season, was a severe blow and he was as important to the Italian side as Argentine World Player of the Year Lionel Messi was to Barca.
"Unfortunately this morning at the end of training Ibra felt a twinge in his adductor and he has remained at home," Allegri told a news conference, adding that the player was also doubtful for the Serie A match at Napoli at the weekend.
"They [Ibrahimovic and Messi] are two champions with different characteristics but within the two teams they have the same value," he said.
"It's a big loss but we have to go out on to the pitch and play regardless."
Both teams come into the match, on the opening matchday of Europe's elite club competition, having drawn 2-2 in their domestic leagues at the weekend.
Seven-times European champions Milan were held at home to Lazio, while Barca, the four-times winners and holders, squandered a two-goal lead at Real Sociedad.
Allegri is also without Italian striker Filippo Inzaghi and Brazil forward Robinho, among others, and may opt to strengthen the midfield and play with only one out-and-out forward in Robinho's international team-mate Alexandre Pato.
The former Cagliari and Napoli midfielder has Antonio Cassano available, the Italy forward who had a brief stint with Barca's great Spanish rivals Real Madrid.
SHATTERED DREAM
Although he has recently sounded a more conciliatory note, Ibrahimovic criticised Guardiola after the Catalan club decided to sell him, calling the former Barca and Spain midfielder "the philosopher who has shattered my dream."
"When I enter a room Guardiola leaves, I don't know if he's scared of me or what," he told reporters last August.
Guardiola said on Monday that it was a shame Ibrahimovic was unable to play as he always liked to see the best players turning out to challenge his side and entertain the fans.
"I don't care what he has said about me, my opinion about him won't change," he told a later news conference.
"Before signing him I thought he was a fantastic player and I still think he is a fantastic player.
"He was fundamental here and I know that he is happy at Milan now and I am very pleased about that.
"I would have preferred to play against him because it's always better when your opponent has all their best players.
"People come to the stadium to see the great players. Ibrahimovic is one of them."
Milan's Netherlands midfielder Mark van Bommel appeared with Allegri at the Italian club's news conference and was bombarded with questions from the Spanish media about what they portrayed as his brutal performance in last year's World Cup final.
The match was littered with fouls and yellow cards and Van Bom
