Iniesta out of Milan meeting
Iniesta sustained the injury in Saturday's 4-0 La Liga win against Real Zaragoza and is being rested as a precaution, Barca said on their website.
He joins suspended full-back Daniel Alves and fellow casualties Adriano Correia and Ibrahim Afellay on the sidelines.
There was better news for coach Pep Guardiola concerning forward Pedro, who has recovered from an ankle problem and is in the squad for the trip to Italy, Barca added.
Group leaders Barca and second-placed Milan qualified for the knockout round with two games to spare and a win for the Spanish club at the San Siro will secure them top spot and a potentially easier passage in the last 16.
Barca have 10 points from four matches and Milan eight.