Iniesta limped out of the match in the 38th minute to be replaced by Cesc Fabregas and subsequent tests have revealed the Spain international suffered a hamstring tear.
The news about Iniesta adds injury to insult for the European and Spanish champions after they conceded a stoppage-time equaliser as Thiago Silva headed in a stoppage-time equaliser.
Milan had also started the match in stunning fashion with Alexandre Pato netting in the opening 30 seconds but Barca turned the game around and looked to be on course to claim the win thanks to goals from Pedro Rodriguez and David Villa.
However, Pep Guardiola’s men could not find a third goal and they were stunned two minutes into injury time when Thiago Silva powered home Clarence Seedorf’s corner to salvage a point for seven-time European champions Milan.
In the other game in the group, Czech champions Plzen shared the spoils with BATE Borisov of Belarus. The Czechs took the lead through Marek Bakos’s 45th-minute goal but Renan Bressan equalised for BATE on 69 minutes to leave all four sides equal in the group.
Genk and Valencia shared the points as their clash in Group E finished in a 0-0 draw at the KRC Genk Arena. The result leaves the two teams in joint second place in the standings, two points behind Chelsea who were 2-0 winners over Bayer Leverkusen.
Marseille top Arsenal’s Group F after they claimed their first win of the season, at Olympiacos. Lucho Gonzalez’s second-half strike gave them a deserved victory in Piraeus.
In Group G, Apoel Nicosia scored twice in three second-half minutes to stun Zenit St Petersburg and open Group G with a deserved win.
Gustavo Manduca scored one and created the 75th-minute winner for Ailton as the hosts recovered from conceding to Konstatin Zyryanov against the run of play.
In the group’s other game, Hulk made up for missing a penalty to score a vicious free-kick as Porto came from behind to launch their 2-1 victory over Shakhtar Donetsk.
The Brazilian hit the bar before seeing his spot-kick come back off the post, only for Shakhtar to take the lead when Adriano pounced on an awful error by Porto goalkeeper Helton.
However, Hulk made amends when he sent an unstoppable shot past Olexandr Rybka. Pinheiro Kleber then tapped home in the second half in a match which saw Shakhtar finish with nine men, Yaroslav Rakitskiy and Dimitro Chygrynskiy both sent off.
* In England last night, Crystal Palace unearthed another young star as 17-year-old Jonathan Williams helped sink Wigan 2-1 in the Carling Cup.
The Wales U21 international curled in a memorable strike to add to Darren Ambrose’s opener as Palace grabbed a deserved cup scalp. Former Eagle Ben Watson pulled one back in stoppage time, but by then it was too late.