The Spain midfielder had to be withdrawn with just under 20 minutes remaining in Barca's 3-1 La Liga defeat against arch-rivals Real Madrid at the Santiago Bernabeu on Saturday.
Iniesta pulled up holding his right leg and signalled to the bench that he was unable to continue.
The influential playmaker, who was replaced by Sergi Roberto, was assessed on Sunday and the Catalan giants confirmed he had damaged his calf.
Barca led through Neymar's early goal, but Cristiano Ronaldo pegged them back from the penalty spot and further goals from Pepe and Karim Benzema consigned Luis Enrique's side to their first La Liga defeat of the season.
Lionel Messi spurned a golden opportunity to double Barca's lead before Real hit back and Xavi was left to rue that miss.
"We didn't lose control at all; when they went up they were more comfortable." he said.
"When we were up 1-0, we should have taken longer possessions. We just missed out on the second goal, and when you miss those in the Bernabeu you pay a hefty price.
"After their second goal, they stayed back and looked for counter-attacks."
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