He said that Mukherjee was correctly upset as he was senior to him and had “knack to tackle issues”.
"Pranab knew I had no choice and any part in this decision," Manmohan Singh said.
“He had a reason to feel upset but he respected me and we have a great relationship which will continue till we live," added ex-PM.
He was speaking at the launch of former President Pranab Mukherjee's new book, 'The Coalition Years: 1996-2012,' in Delhi.
Singh, who was an economist and Rajya Sabha MP was chosen to be the Prime Minister of India in 2014 after Sonia Gandhi sacrificed the top office job.
Sonia’s decision came as a surprise for many in the party and Opposition as she chose a junior Manmohan Singh over more experienced Pranab Mukherjee.
Praising Pranab Mukherjee, former prime minister of the country said, “He is a politician by choice and it comes naturally to him... I became a politician by accident."
However, Pranab Mukherjee who became President in 2012 had said at the time of his nomination for country’s top constitutional post that he was not upset after being overlooked for Prime Minister’s job.
"Why should I regret? The party President has trusted me, has considered me capable of discharging certain responsibilities, by me alone. That is a great trust," Mr Mukherjee had said during a media interview.