Ravindra Jadeja batted ahead of Iyer on the third day, and while that wasn't unusual - India often use the left-handed Jadeja to break up a largely right-handed top order - Iyer didn't come out to bat even when Jadeja was dismissed on the fourth morning. Instead, wicketkeeper-batter KS Bharat walked in at No. 6 and then, in the second session, Axar Patel replaced Bharat.
The BCCI sent out this statement shortly after: "Shreyas Iyer complained of pain in his lower back following the third day's play. He has gone for scans and the BCCI Medical Team is monitoring him."
It is not yet clear if Iyer would be able to bat in this Test match at all.