Iniesta was given permission to sit out Barcelona's training session on Thursday by head coach Ernesto Valverde.
The club have not given a reason for the 33-year-old's midfielder's absence.
Iniesta played in Barcelona's 5-0 win over Chapecoense in the Joan Gamper Trophy on Monday.
The Spain great endured injury absences last season due to knee, calf and adductor problems.
Barca welcome Madrid to Camp Nou for the first leg of the Spanish season's traditional curtain raiser, before heading to the Santiago Bernabeu for next Wednesday's return.