"Nobody else can occupy it (PM's post)," he told reporters while responding to questions about whether Modi would return to power in 2019.
"Now nobody has the strength to take on the prime minister," he said.
Quizzed about his future role in national politics, Kumar, who heads the JD(U) and was earlier seen as a potential challenger to Modi in 2019, said, "Ours is a small party which does not harbour big national aspirations."
When reminded of his remark while breaking away from the NDA in 2013 that he would "rather be decimated than join hands with BJP", he said, "That was in the context of that time."
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Kumar spoke at length about events leading to the break-up of the Grand Alliance and his accepting the proposal from the "highest level" in the BJP for joining hands to form the government.
He also hit back at RJD supremo Lalu Prasad and Congress vice president Rahul Gandhi for criticising him.
"He (Lalu) arrogantly says that he made me the CM ... The people of Bihar showed him his worth in 2010 (when RJD's strength was reduced to 22)."
"The decision (to dump the alliance) did not come overnight. I repeatedly swallowed objectionable comments from the other side (RJD), including one by one of its leaders (former MP from Siwan Mohammad Shahabuddin who called him 'a CM of circumstance'). Lalu Prasad ignored it, saying it is an internal issue of his party."
But, Kumar said, once the issue of corruption came to the forefront with CBI registering an FIR against Deputy Chief Minister Tejashwi Yadav in a graft case, he could not carry forward the alliance any more.
"I, while supporting demonetisation, had made a strong case for simultaneous hit on benami property. I had no ground to defend them."
He ridiculed Prasad's secular credentials. "Making huge money hiding behind the shield of secularism ...Is this secularism? ... I need no certificate of secularism from anybody."
Kumar said his government had given compensation to the victims of the Bhagalpur riots on par with those of the 1984 anti-Sikh riots.
Hitting out at Lalu for his repeated claim that he made him the chief minister despite his party having more MLAs than JD(U), Kumar said it reflected the RJD leader's "arrogance".
Recounting his association with Lalu, Kumar spoke of how he helped him win the election of the Patna University Students Union president. He also spoke about having mobilised non-Yadav MLAs to back Lalu's chief ministerial bid in 1990.
On reports that senior JD(U) leader Sharad Yadav was upset over the party joining hands with BJP, he said, "It is not necessary that everybody always agrees on everything. One can have divergent views. The decision to break the Grand Alliance was taken by Bihar JD(U) at its executive meeting which I had to abide by."
He mocked at Rahul Gandhi for claiming that he had an inkling that Kumar would walk out of the coalition for the last three to four months. "Then why did he (Gandhi) meet me when I had gone to Delhi and sought his intervention...To ask RJD to come clean on the accusations.
"Congress did not act on time in Assam also when AGP had come on board. It cost us the Assam polls," he said, adding "we can be a partner but not a camp follower."