The Grand Alliance ministry, headed by Kumar, collapsed in July last year after the chief minister resigned over graft charge against his then deputy, Tejashwi Yadav.
Subsequently, Kumar joined hands with the BJP to form another coalition government in Bihar.
"The very day the Mahagathbandhan government was installed, I had said to those close to me that this was not going to last for more than a year-and-a-half. I knew it from the very beginning. I still managed to carry on for 20 months," the JD(U) chief told reporters here on the sidelines of his weekly "Lok Samvad" (public interaction) programme.
The Grand Alliance was formed shortly after the 2014 Lok Sabha polls when Kumar and Prasad decided to bury the hatchet.
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Both the JD(U) and the RJD had suffered a severe drubbing in the Parliamentary election, which saw the BJP putting up its best ever electoral performance, riding on a strong wave in favour of Narendra Modi.
However, Kumar's statement drew flak from former JD(U) president Sharad Yadav, who had raised a banner of revolt when the Bihar chief minister joined hands with the BJP.
Yadav was disqualified from the Rajya Sabha a couple of months ago, following an application by the JD(U), which had held that the former party president's presence at rallies and meetings organised by the RJD-Congress combine were tantamount to his having "voluntary renounced" the membership of the party.
"The respected 67-year-old should be ashamed that he made a 28-year-old an excuse for his shenanigans. Uncle, you could have parted ways like a man," Yadav, who is the younger son of Lalu Prasad, said.