Sources said Prasad made the proposal at a meeting attended by JD-U President Sharad Yadav and Samajawadi Party chief Mulayam Singh Yadav, called to remove irritants in the way of a merger before the Bihar polls due later this year.
Kumar did not attend the meeting despite being in Delhi. His aides claimed he had undergone a "minor eye surgery" and has been advised rest.
Sources said Prasad wants all non-NDA parties, including the Left, to form a grand alliance to defeat BJP in the state.
While JD-U is pushing for a quick formalisation of the merger of six Janata Parivar offshoots, which was announced last month, SP and RJD are having second thoughts.
SP believes it has little to gain from any such merger with many of its leaders fearing a loss of stature in the new party, while Prasad apprehends Kumar will steal a march over him in Bihar despite his party having a much bigger support base than JD-U.
For the record, Prasad said attempts at unity are on and brushed side reports that Kumar was unhappy with the recent turn of events. "I have no differences with anybody," he said.
Manjhi, who was chief minister before Kumar ousted and replaced him after he refused to step down voluntarily, is said to be veering towards an alliance of his party Hindustani Awam Morcha with BJP and has made it clear that he would not be part of any formation involving Kumar.
Prasad also attacked BJP over the celebration of noted Bihar poet Ramdhari Singh Dinkar's works, saying it was a political move, a reference to the ruling party's attemt to woo the influential Bhumihars, Dinkar's caste.
"Prime Minister Narendra Modi speaks about shedding casteism, while BJP holds rallies of all castes in Bihar. They have nothing to show after one year in power," he said.