India, March 19 -- For Keki Mistry, it's a return to the helm born out of sudden necessity. Doesn't matter if it's only for three months. The man who built India's largest mortgage financier over four decades is now interim chairman of a combined HDFC Bank Ltd.

Following the shock resignation of part-time chairman Atanu Chakraborty over what he said were "ethical concerns", the Reserve Bank of India swiftly approved Mistry as the interim chairman of the "systemically important bank". He got to work immediately. Less than 12 hours after the former bureaucrat's exit was made public, he was speaking to the media to calm the nerves of investors and customers alike, even as the stock of India's largest private bank tumbled.

"There could have...