The midfielder picked up a thigh injury in Saturday's 4-0 Liga win over Zaragoza and Barca have now revealed on their official website that Iniesta has not been included in the match squad for the crunch clash at San Siro.
The 27-year-old came off the bench after 56 minutes and lasted until the final whistle, but complained about pain in his left thigh afterwards and will therefore not feature against Milan.
Wednesday's match at San Siro kicks off at 20.45CET. Barcelona have a two-point lead over the Rossoneri at the top of the group.
"Starting last Friday and continuing every Friday through to January, Goal.com will publish extracts from Graham Hunter's highly-anticipated book, Barca: The Making of the Greatest Team in the World, which kicked off with a fascinating segment on Lionel Messi's rise to the top at Camp Nou. Don't miss it!"
How do you stay up with football when on the move? With http://m.goal.com –your best source for mobile coverage of the beautiful game.