Iniesta was replaced by Spain team-mate Cesc Fabregas shortly before half-time at the Nou Camp, before Barca conceded a stoppage-time goal to draw 2-2 with the seven-times European champions.
"Tests have confirmed that the player suffered damage to the femoral biceps of the left leg," the La Liga club said in a statement on their website.
"The approximate time he will be out of action is four weeks," they added.
Iniesta joins injured pair Gerard Pique and Alexis Sanchez on the sidelines.
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