Sir,
Accident Report on the prescribed form is required to be submitted to the appropriate Branch Office of ESIC, even if the Insurance Number has not been allotted. Delay is always difficult to justify later on. You can submit the Declaration Form of the injured employee online or by the old procedure, as the Branch Office may accept it. In the Accident Report, in the column of the Insurance Number of the employee, you can indicate "Applied For."
I also hope that you will make compliance in respect of other issues/points - viz. entry in the Accident Register, intimation of the accident to the Inspector of Factories, and in case of a serious accident, a Report to the appropriate Police Authorities (FIR).